Objection 1. It would seem that respect of persons does not take place in showing honor and respect. For honor is apparently nothing else than "reverence shown to a person in recognition of his virtue," as the Philosopher states (Ethic. i, 5). Now prelates and princes should be honored although they be wicked, even as our parents, of whom it is written (Exodus 20:12): "Honor thy father and thy mother." Again masters, though they be wicked, should be honored by their servants, according to 1 Timothy 6:1: "Whoever are servants under the yoke, let them count their masters worthy of all honor." Therefore it seems that it is not a sin to respect persons in showing honor.
Objection 2. Further, it is commanded (Leviticus 19:32): "Rise up before the hoary head, and, honor the person of the aged man." But this seems to savor of respect of persons, since sometimes old men are not virtuous; according to Daniel 13:5: "Iniquity came out from the ancients of the people [Vulgate: 'Iniquity came out of Babylon from the ancient judges, that seemed to govern the people.']." Therefore it is not a sin to respect persons in showing honor.
Objection 3. Further, on the words of James 2:1, "Have not the faith... with respect of persons," a gloss of Augustine [Ep. ad Hieron. clxvii.] says: "If the saying of James, 'If there shall come into your assembly a man having a golden ring,' etc., refer to our daily meetings, who sins not here, if however he sin at all?" Yet it is respect of persons to honor the rich for their riches, for Gregory says in a homily (xxviii in Evang.): "Our pride is blunted, since in men we honor, not the nature wherein they are made to God's image, but wealth," so that, wealth not being a due cause of honor, this will savor of respect of persons. Therefore it is not a sin to respect persons in showing honor.
On the contrary, A gloss on James 2:1, says: "Whoever honors the rich for their riches, sins," and in like manner, if a man be honored for other causes that do not render him worthy of honor. Now this savors of respect of persons. Therefore it is a sin to respect persons in showing honor.
I answer that, To honor a person is to recognize him as having virtue, wherefore virtue alone is the due cause of a person being honored. Now it is to be observed that a person may be honored not only for his own virtue, but also for another's: thus princes and prelates, although they be wicked, are honored as standing in God's place, and as representing the community over which they are placed, according to Proverbs 26:8, "As he that casteth a stone into the heap of Mercury, so is he that giveth honor to a fool." For, since the gentiles ascribed the keeping of accounts to Mercury, "the heap of Mercury" signifies the casting up of an account, when a merchant sometimes substitutes a pebble ['Lapillus' or 'calculus' whence the English word 'calculate'] for one hundred marks. So too, is a fool honored if he stand in God's place or represent the whole community: and in the same way parents and masters should be honored, on account of their having a share of the dignity of God Who is the Father and Lord of all. The aged should be honored, because old age is a sign of virtue, though this sign fail at times: wherefore, according to Wisdom 4:8-9, "venerable old age is not that of long time, nor counted by the number of years; but the understanding of a man is gray hairs, and a spotless life is old age." The rich ought to be honored by reason of their occupying a higher position in the community: but if they be honored merely for their wealth, it will be the sin of respect of persons.
Hence the Replies to the Objections are clear.
Source: Summa Theologica (New Advent)