But why dwell longer on the reverence and sacred respect of Christians to the emperor, whom we cannot but look up to as called by our Lord to his office? So that on valid grounds I might say Cæsar is more ours than yours, for our God has appointed him. Therefore, as having this propriety in him, I do more than you for his welfare, not merely because I ask it of Him who can give it, or because I ask it as one who deserves to get it, but also because, in keeping the majesty of Cæsar within due limits, and putting it under the Most High, and making it less than divine, I commend him the more to the favour of Deity, to whom I make him alone inferior.
But I place him in subjection to one I regard as more glorious than himself. Never will I call the emperor God, and that either because it is not in me to be guilty of falsehood; or that I dare not turn him into ridicule; or that not even himself will desire to have that high name applied to him. If he is but a man, it is his interest as man to give God His higher place. Let him think it enough to bear the name of emperor. That, too, is a great name of God's giving. To call him God, is to rob him of his title.
If he is not a man, emperor he cannot be. Even when, amid the honours of a triumph, he sits on that lofty chariot, he is reminded that he is only human. A voice at his back keeps whispering in his ear, “Look behind you; remember you are but a man.” And it only adds to his exultation, that he shines with a glory so surpassing as to require an admonitory reference to his condition. It adds to his greatness that he needs such a reminiscence, lest he should think himself divine.
Source: The Apology (New Advent)