Augustus, the founder of the empire, would not even have the title Lord; for that, too, is a name of Deity. For my part, I am willing to give the emperor this designation, but in the common acceptation of the word, and when I am not forced to call him Lord as in God's place. But my relation to him is one of freedom; for I have but one true Lord, the God omnipotent and eternal, who is Lord of the emperor as well. How can he, who is truly father of his country, be its lord? The name of piety is more grateful than the name of power; so the heads of families are called fathers rather than lords.
Far less should the emperor have the name of God. We can only profess our belief that he is that by the most unworthy, nay, a fatal flattery; it is just as if, having an emperor, you call another by the name, in which case will you not give great and unappeasable offense to him who actually reigns?— an offense he, too, needs to fear on whom you have bestowed the title. Give all reverence to God, if you wish Him to be propitious to the emperor. Give up all worship of, and belief in, any other being as divine.
Cease also to give the sacred name to him who has need of God himself. If such adulation is not ashamed of its lie, in addressing a man as divine, let it have some dread at least of the evil omen which it bears. It is the invocation of a curse, to give Cæsar the name of god before his apotheosis.
Source: The Apology (New Advent)