I need to make a video for a history class on the Roman empire and the rise of Christianity, but there are many things that I cannot find. If you answer, thank you.
Dominus, plural Domini, in ancient Rome, “master,” or “owner,” particularly of slaves. The name later became the official title for the emperor, beginning with Diocletian, who reigned from ad 284 to 305.
The mutual relation of Slave and Master among the Romans was expressed by the terms Servus and Dominus; and the power and interest which the dominus had over and in the slave was expressed by Dominium. The term Dominium or ownership, with reference to a slave, pointed to the slave merely as a thing or object of ownership, and a slave as one of the Res Mancipi classed with other objects of ownership.
Sources and suggested reading:
The Institutes of Justinian: With English Introduction, Translation, and Notes By Thomas Collett Sandars
Words like kurios and dominus are literary words that would be not normally be used in everyday speech by a slave. Plautus most famous character, the slave Epidicus, addresses his master as "ere", which is Greek slang and means "boss". Another of Plautus' slaves, Pseudolus, uses exactly the same word to address his master.
If you read "Latin Forms of Address: From Plautus to Apuleius" by Eleanor Dickey it confirms my answer: slaves almost always call their master/mistress erus/era to their face. Note that "ere" is the vocative form, so that is the form that would normally be used. You will sometimes find the word in Latin dictionaries under "herus". Don't be fooled by some of these dictionaries into thinking this is an "official" Latin word, its actually very colloquial Greek.
A greek-speaking slave (δούλος, doulos) I think would call his master κύριος (translit: kurios; lord, master, sir; vocative form: κύριε).
Roman was not the only language spoken in the Empire, especially amongst lower classes. Alexander's "Koine" (Common) Greek might have been more common, certainly in the early empire. The early church fathers wrote in Greek from the first to the third century, and fourth century fathers (e.g. Ambrose, Augustine) are familiar with Greek (and the Nicene Creed was drawn up in Greek), though the language waned -- in the Western part of the empire only -- over the later part of the fourth century.
There's a short and insightful book on the Rise of Christianity by Rodney Stark.