Which religion was the first monotheistic one? Was it Judaism or a religion which disappeared from practice?
Judaism is very old, but it was not originally monotheistic (see below).
An earlier instance of monotheistic or monotheistic-esque worship occurred in the form of Atenism, the worship of the deified sun-disk Aten in Ancient Egypt. The Pharaoh Amenhotep IV (later Akhenaten), reigning around 1353/1351-1336/1334 B.C., promoted it as an arguably monotheistic state religion for Egypt.
O sole god, like whom there is no other! Thou didst create the world according to thy desire, Whilst thou wert alone: All men, cattle, and wild beasts (...) The lord of all of them, wearying (himself) with them, The lord of every land, rising for them,
Worship of Aten predates Akhenaten, but under his rule Atenism morphed from a more traditional henotheism into something that could be recognised as monotheism. He first elevated Aten into the supreme god, and later declared Aten to be the only god. He seemed to have also banned the worship of other gods and idols. However, soon after Akhenaten's death, the previous cult of Ra was restored and Atenism came to an end.
Atenism under Akhenaten is usually cited as the first true monotheism, but it might have been inspired by earlier Egyptian thoughts. The Lord Carnarvon (1866-1923) has written:
It has been claimed by some that Amenhetep IV was the first monotheist in Egypt, but the acceptance of this statement depends upon what meaning is given to the word monotheism, i.e., the doctrine of there being only one god. The passages from the Moral Papyri quoted above show that the Egyptian priests and learned men were monotheistic, even though they do not proclaim the oneness of the god to whom they refer.
- Herbert, George Edwarde Stanhope Molyneux, and Earl of Carnarvon. "Amenism, Atenism and the Egyptian Monotheism."
In any case, Atenism's rise and demise predates the emergence of Judaism as a monotheistic religion by several hundred years.
Well, I did not think this was controversial; given @TylerDurden's reaction, apparently I'm mistaken, so here's some more elaboration:
The Jewish faith did not fully commit to monotheism until around the time of the Babylonian Captivity. Prior to that, the Jewish people were largely henotheistic, if not polytheistic. This is not at all a new concept.
A survey of the development of Judaism from polytheism, henotheism and monotheism to the universality of God is in order. (...) The Jews of [the First Temple period] were henotheists; they worshiped their God, the God of the land who fought their enemies and was supreme over other gods. From time to time they worshiped gods of different countries.
The calamities which befell the Judaeans in the burning of the First Temple and in their exile to the shores of the rivers Tigris and Euphrates had a chastening effect upon them, and this may be one of the causes which revolutionized their relationship to their God. Before the exile the Jews regarded their God as the God of the land and superior to other gods, but they also worshiped other deities.
With the return of the Jews from Babylonia to Judaea, first under the leadership of Zerubbabel and Joshua and later under Ezra and Nehemiah, henotheism disappeared and monotheism took its place.
- Zeitlin, Solomon. "Judaism as a Religion: An Historical Study. XI. Religion and Nationality (Continued)." The Jewish Quarterly Review (1944): 179-225.
That the switch to monotheism occurred around the same time the Jews were deported to Babylon is a mainstream view.
While some scholars continue to adhere closely to the biblical story, in which the ancestors of Israel introduced a pure monotheism at the beginning of the nation's history, the dominant tendency nowadays is to presuppose a lengthy development in Israel's religion from an originally polytheistic or henotheistic to a monotheistic system by the time of the Babylonian Exile.
Although the Shema in its original 7th century BCE context may have meant that among all the gods, the Judaeans should only worship YHWH (henotheism), Judaism has come to understand the Shema as its central declaration of faith in one indivisible god (monotheism).
- Ehrlich, Carl S. Judaism. The Rosen Publishing Group, 2010.
Even those who might not agree, recognises this is the consensus view.
Most scholars whose work focuses on Israelite religion recognize that the Hebrew Bible contains a number of references assuming and even affirming the existence of other gods. As a corollary to this observation, scholars also frequently assert that no explicit denial of the existence of other gods occurs until the time of Deutero-Isaiah and thereafter in a presumed campaign by zealous scribes to expunge such references from the sacred text. Even the Shema and the first commandment do not consign the other gods to fantasy, since the demand is made that no other gods should be worshiped. The data apparently informs us that Israelite religion evolved from polytheism to henotheistic monolatry to monotheism.
- Heiser, Michael. "Monotheism, Polytheism, Monolatry, or Henotheism? Toward an Assessment of Divine Plurality in the Hebrew Bible." Faculty Publications and Presentations (2008): 277.
Of course, one could get around all this by defining Judaism as only beginning when the Jews conclusively rejected the existence of any other god. Though that would be more of a semantics quibble really. As is arguing about "single-ruler cults" vs "relegions (sic)".
In an appendix to his book "When Our World Became Christian", Paul Veyne studies the extent to which the concept of monotheism applies to Judaism. His main point is that the concept of monotheism ("there is only one God") can be differentiated from monolatrism ("ye shall worship only one God") only if the idea of "a non-existent deity" can be conceptualized. That idea appeared at one time; before that invention, people did not think of gods in terms of true/fake, but rather in terms of yours/others, or stronger/weaker. Paul Veyne references the relevant Scripture texts in which the transition from monolatrism to monotheism can be seen.
Apparently, the Babylonian captivity was a turning point because it exposed the Hebrew intellectual elites to concepts which were developed in more Eastern areas, notably Zoroastrianism. Though Zoroastrianism gained the status of "state religion" in Persia only later on, the basic concepts were older (the exact date is highly disputed, but the current consensus points to "some time in the 2nd millenium BC"). The ideas had begun to percolate to neighbouring Babylonia at the time the Hebrew were there. Among these concepts was the notion that there was a Supreme Deity (Ahura Mazda), and other "gods" were really subordinates, even proxies; every prayer sent to any god was ultimately brought to the attention of Ahura Mazda. In that sense, the other gods in Zoroastrianism were already at that time beginning to be perceived as proto-angels and demons, to be respected and/or feared, but not "gods" in the same sense of Ahura Mazda.
When the Hebrew came back from Babylon, monotheism crystallized in their minds: they now understood that a god could be fake, non-existent. This contrasts with what philosophers were thinking in 5th century BC Greece: for them, the "divinity" was basically the Cosmos, the order. Conceptual gods were a-plenty, e.g. Eros (for love) or Chronos (for time). They had no notion that a concept could be non-existent: if you can think about it, then it exists, and is part of the Cosmos, thus you can worship it.
Since monotheism was a gradual innovation, there are "intermediate states" and one cannot really pinpoint an exact year in which it happened. The term henotheism has been coined to describe these intermediaries. In the case of Europe and Middle-East, it seems that true monotheism emerged with Judaism in the 6th or 5th century BC, although some definitions of monotheism can include earlier Zoroastrianism or Atenism (as described by @Semaphore). (However, it can be said that when Akhenaten mandated what was effectively monolatry, he was more concerned about the mundane power of the Amun priesthood than the alleged fakeness of the other gods.)
Although we usually recognise Judaism as the first monotheistic faith, the title may actually go to Zoroastrianism. Zoroastrianism was established around the 6th century BCE. In a nutshell, it abandoned the previous Persian pantheon and simplified it to
"two forces Spenta Mainyu (Progressive mentality) and Angra Mainyu (Destructive Mentality) under the one God, Ahura Mazda (Illuminating Wisdom)"
Although Judaism may be older than the 6th century BCE, Judaism was not a strictly monotheistic faith until the return of the Israelites from the Babylonian exile, which happens to be around the 6th century, from Persia.
Zoroastrianism survived as the state religion of Persia for a few centuries until Alexander the Great's arrival, after which it gradually declined. Most of the remaining Zoroastrians converted to Islam when it arrived (likely simplified by the monotheistic similarities), although there's still a lively community of Zoroastrians, many in Iran (150'000 - 2 million depending on who you ask).
In Egypt, Pharaoh Amenhotep IV started a new monotheistic religion and renamed himself Akenaten, moving to a new capital city Armana unsullied by the normal religion. This would be about 1350 BC to 1320 BC.
When he died, his son Tutankhamun reconciled with the old regime, and the city was abandoned. This heresy and the need to wipe out its existence is one reason why King Tut's tomb was left intact so that it could be found in the 1920s.
I know Sigmund Freud wrote a book "Moses and Monotheism" trying to tie Judaism's development to Akenaten, but most seem to be unconvinced.
A pity question. Atenism was a sect that departed from the traditional polytheistic Egyptians but did not really catch on after the death of Amenhotep. One, as mentioned above, can make a case for its brief appearance being the first recorded monotheistic belief system. Also often overlooked is that Judaism as practiced during the first temple period was henotheistic, i.e., recognizing the existence of other deities while worshipping YHWH or El as the primary regional or ethnic deity. Judaism changed and became more monotheistic after the Mesopotamian diaspora due to the influence of Zoroastrianism under the Persians.
Let me give an input regarding Hinduism which is considered as a major polytheistic religion and one of the oldest in the history.
Hinduism is actually have another name, 'Sanathana Dharma', which is considered as the culture of India. Even though Hinduism now considered as a religion with all essence of this culture.
There are more than 33 Million Gods of Hinduism. Why Hindus worship so many gods and goddesses is a real mystery for most people. What is the role of Hinduism in a monotheistic religion question in here? It do have an important role because the ultimate message from Hinduism lead to the concept of single God or all Gods are the same. Even if there are so many Gods like Trinity Shiva,Vishnu and Brahma, the core of the Hinduism says that all these Gods are the same. Krishna,Rama, Durga, Ganesha,etc and the number is lot more in the list of Gods. But see what Vedas says,
The most important texts in Hinduism,4 veda's mahavakyas(ultimate messages) are the following
1.Rig Veda - prajñānam Parabrahma - Wisdom/consciousness is the parabrahma(highest truth, not to be confused with 'Brahma' in Trinity)
2.Atharva Veda - ayam ātmā brahma- I am this Self is parabrahma
3.Sama Veda - tat tvam asi- You are that(parabrahma)
4.Yajur Veda - aham brahmāsmi- I am that (parabrahma)
All these creators in the world is considered as the part of one ultimate truth which can be called as a 'monotheistic GOD'. According to Hinduism, all these 33 million Gods are the same. Most of these Gods are the incarnations. Like Krishna and Rama are the incarnation of Lord Vishnu. Anyone can worship any of these Gods and get to the ultimate truth.
See a related question in Hinduism.SE here.
Also Hindus does not have strict restrictions in going to church or other temples like in Christianity or Islam. The reason is that Hindus believe that all Gods(in other religions too) are the part of the one ultimate truth or God. So isn't it the monotheism? Hindus who know the message of their culture know that everything is one.
Hinduism is one of the oldest religion, so while considering the core of message of the hinduism, it can be considered as the first monotheistic religion. It is way too older than Judaism, zourastrism or any other religions mentioned in other answers.
It is actually a chronological tie between Judaism and the religion of Ismail, Abraham's eldest son.
As I had mentioned in an earlier post, Ismail and his Father Abraham either built or commissioned the building of the Kaaba in Arabia-(specifically, in the city of Mecca). The Kaaba was built as the First physical structure or House of worship dedicated to honoring, worshiping and praying to a single, abstract-(though communicative) God in world history 1000 years before the construction of The Temple of Solomon in Jerusalem. Ismail's monotheism-(which was nearly indistinguishable from his younger brother Isaac's monotheism), was short lived/ ephemeral. After Ismail's death, the vast majority of his descendants and future descendants gradually drifted away from Abrahamic Monotheism and adopted various deities that were depicted in idolatrous form. The one exception were the Hanifs, who steadfastly maintained and retained the Abraham-Ismail monotheistic legacy and tradition for 2500 years, until Muhammad's time.
However, it is Judaism that has maintained its steadfast loyalty and dedication to the honor, worship and prayer to a single, abstract-(though communicative) God, beginning with Abraham and Isaac, though furthered by Jacob, Joseph and a long line of Prophetic figures, such as Moses, Aaron, Joshua, Elijah, Ezekiel, Daniel and Isiah. Judaism isn't necessarily the oldest or first monotheistic religion in world history; though Judaism, is, by far, the longest lasting and historically consecutive monotheistic religion in world history dating back 4000 years.
This does depend on the definition being used for monotheism, which there appears to be some confusion about. Originally monotheism implied there was only one God, and all religions were searching for that God (possibly erring in their perception of God). In this sense, Judaism is not monotheistic, as it says there is one God, but other religions are NOT searching for God, but are worshiping false gods.
In the sense of the belief of there only being one God, however, the oldest recorded monotheistic religion would be Judaism and its precursors. The religion describes only one God, the mention of other deities being restricted purely to idols and to poetic script.
Some have tried to say Atenism is the oldest religion, from 1350 BC to 1320 BC. The religion, however, did acknowledge other gods, but some have excused that detail for political reasons, the king being urged to admit other gods.
Judaism never acknowledged any other gods, but is sometimes excused of this on the grounds the Jewish people often practiced multiple religions. No doubt there were worshipers of Aten who also believed in other gods, so I believe this point to be inadmissible.
Dating Judaism and Moses
You can take several possible dates for the beginning of the religion. Judaism itself is taken from Judah (יהודה), and the modern form of the religion (containing all the books of the Tanakh) would've began around 516 BC.
The date of Moses formalizing the religion is commonly stated as 1250–1200 BC. The biblical record dates it as 1450 BC, which agrees with several archaeologists' findings. Several theories would push this date back to 1650 BC, contemporary with the Bronze Age Collapse of the Middle Kingdom of Egypt and the sacking of Canaan. (Source: Patterns of Evidence: Exodus)
Regardless of how you date Moses, Abraham predates Atenism by centuries. There is no evidence to suggest Abraham believed there were many gods, the only accounts we have of him suggesting he believed in one God. The genealogy of the bible is a good example of this, where he and others recorded a lineage back to the first man, Adam, who himself was a monotheist as described in the religion.
The fact these beliefs are contemporary back to the time of Abraham is confirmed by similar folk history, legend and religion across the world. Many people talk of a first man, of a great flood that wiped out the Earth, of eight people who survived on a boat and repopulated the world, from Polynesia to China.
This would make Judaism and its precursors the most likely candidate for the first monotheistic religion.