Thomas Aquinas

“ Now servitude is due to God and to man under different aspects: even as lordship is competent to God and to man under different aspects. For God has absolute and paramount lordship over the creature wholly and singly, which is entirely subject to His power: whereas man partakes of a certain likeness to the divine lordship, forasmuch as he exercises a particular power over some man or creature. Wherefore dulia, which pays due service to a human lord, is a distinct virtue from latria, which pays due service to the lordship of God. ”

“ Or we may reply that the movement towards an image is, after a fashion, towards the thing, yet the movement towards the thing need not be towards its image. Wherefore reverence paid to a person as the image of God redounds somewhat to God: and yet this differs from the reverence that is paid to God Himself, for this in no way refers to His image. ”

“ In every man is to be found something that makes it possible to deem him better than ourselves, according to Phil. 2:3, "In humility, let each esteem others better than themselves," and thus, too, we should all be on the alert to do honor to one another.
Reply to Objection 4: Private individuals are sometimes honored by kings, not that they are above them in the order of dignity but on account of some excellence of their virtue: and in this way Tobias and Mardochaeus were honored by kings.
Art. 3 - Whether dulia is a special virtue distinct from latria? ”

“ Before God, Who is the searcher of hearts, the witness of one's conscience suffices. wherefore honor, so far as God is concerned, may consist of the mere internal movement of the heart, for instance when a man acknowledges either God's excellence or another man's excellence before God. But, as regards men, one cannot bear witness, save by means of signs, either by words, as when one proclaims another's excellence by word of mouth, or by deeds, for instance by bowing, saluting, and so forth, or by external things, as by offering gifts, erecting statues, and the like. ”