Well, the best examples that comes to mind are Plato and Aristotle.
Both Plato and Aristotle were born and raised in Greece proper and lived during a polytheistic age whereby the Olympian Deities were the central focus on worship, both at Mount Olympus in Northern Greece, as well as individual Gods and Goddesses represented at various Temples throughout the Hellenic landscape; the most famous being The Parthenon in Athens.......a city that was Plato's home, as well as Aristotle's adopted city.
Although the Olympian religion was widespread throughout Greek society during the age of Plato and Aristotle, both Philosophers helped to pioneer their own versions of Monotheism.
In the case of Plato, there is "The Theory of the Forms", whereby the Universe is comprised of both visible and invisible configurations, that is to say, a square or a circle is not merely a square or a circle; rather, a square or a circle has a larger presence that is independent or external to the perceptible and the tangible. In a way, Plato originated the famed, "argument by design", which has often been used by Christian Theologians-(and I believe Islamic Theologians as well) during the Middle Ages to affirm and defend the existence of God. The Platonic "Forms", were not a collection or representation of specific Deities-(such as the Olympian Gods and Goddesses). The "Forms" were Cosmic configurations that were a meaningful and purposeful reflection of the abstract-(though impersonal and non-personified) presence of God. It is this major distinction between abstraction and personalization-(as well as personification) of the Divine that distinguishes Plato from Theism and specifically, the Olympian Polytheism of his time within Greece proper-(as well as throughout the larger Greek world).
Aristotle, who was Plato's student for 20 years-(and probably his best student), had also deviated from the Greek polytheistic norm whereby his philosophy regarding the existence of a single and abstract God went even further.
In his most famous work on the existence of God, "The Metaphysics" primarily focuses on the nature, origin and meaning of causality, though in his final Chapter or "Book", Aristotle essentially defines the existence of God as, "An Immovable Mover" or "Unmoved Mover"-(I don't have the exact Ancient Greek translation, though the Latin translation was, "The Prime Mover", which is the primary description that is used within the West, especially, the English speaking West, to the present-day). For Aristotle, the Universe/Cosmos necessitated an Existing-(and Purposeful) Force which both served as the Origin of All Being, as well as having some Grand purpose which would best explain the deeper meaning of Matter-(or the Materialistic).
Both Plato and Aristotle rejected the earlier Atomic Teachings of the Northern Greek Philosopher, Democritus of Abdera, who coined/invented the word, "Atom" and who essentially believed that the Universe was comprised of atoms that were inherently purposeless, infinite and indestructible-(i.e. "it cannot be created, nor destroyed"). Plato vehemently opposed Democritus'viewpoint and Aristotle, though a Scientifically oriented Thinker such as Democritus, also found Atomism to be extreme in its teaching as well as logically implausible. In Democritus' Universe, there is the absence of a beginning or origin, as well as an absence of God(s) whereas Aristotle's Universe necessitates the existence of a Grand Figure who serves as the Origin of all "Motion". Though paradoxically, such a Grand Figure does not move and is, by its nature, unchanging; (hence, "An Unmoved Mover" who is independent of and external to, the laws of motion, space and time. A pre-Temporal and pre-Spatial Grand Being whose presence legitimates and necessitates the very existence.........of existence)
Both Plato and Aristotle had a Purposeful God-(i.e. "Teleological God"), though both Philosophers, adamantly rejected the personalization and personification of the God. For Plato and Aristotle, the presence of God was not akin to humankind, instead, it was a Grand Force who was devoid of all human attributes and is distinct from humankind itself.
The Platonic and especially, the Aristotelian God, absolutely rejected the notion of a "Revealing God" who, according to various religions of their time, possessed human-like characteristics and attributes-(whether visible or invisible). The so-called, "revelation" of God, according to Plato and Aristotle, may have been the geometric and ontological reflections and patterns displayed in the workings of the Universe.
Both Plato and Aristotle-(and I suspect many of their students at the Academy and Lyceum), held radically different metaphysical philosophies when compared with the centuries old mainstream Olympian religion practiced by the vast majority of Hellenes in Greece proper and elsewhere in the greater Greek world-(such as Anatolia, Sicily and the Southern Italian coast). One must remember that publicly contradicting, defying or denying the existence of the Olympian Deities in their time-(as well as in previous times), was a crime, which could lead to imprisonment or even the death penalty-(i.e. Socrates, who was also Plato's Mentor/Teacher). Plato had some difficulties with the political Authorities in Siracusa, Sicily, though managed to escape unharmed-(this incident may or may not have been related to his views on religion). Though Aristotle, like Socrates before him, was less fortunate. Aristotle, was brought before the Authorities regarding his views on religion, though unlike Socrates, he avoided a trial by escaping from Athens where shortly thereafter he died. Again, even in Liberal and cosmopolitan Athens, blasphemy/apostasy or even a slight deviation from the religious norm within the public square was not acceptable and was a punishable crime.
Of course centuries after Plato and Aristotle, Greece did indeed become a monotheistic society with the advent of Christianity. The aim of ending Olympian paganism-(an idea that both Plato and Aristotle shared), was fulfilled with the nationalization of Christianity within Greece-(and parts of the greater Greek world) under The Edict of Thessaloniki by the Byzantine Emperor Theodosius in 380 AD/CE. However, even though monotheism did supplant polytheism within Greece, would Plato and Aristotle accept Christian monotheism? It is difficult to say, but my instincts say......probably not.