Objection 1. It would seem that respect of persons is not a sin. For the word "person" includes a reference to personal dignity [Cf. I, 29, 3, ad 2]. Now it belongs to distributive justice to consider personal dignity. Therefore respect of persons is not a sin.
Objection 2. Further, in human affairs persons are of more importance than things, since things are for the benefit of persons and not conversely.
But respect of things is not a sin. Much less, therefore, is respect of persons.
Objection 3. Further, no injustice or sin can be in God. Yet God seems to respect persons, since of two men circumstanced alike He sometimes upraises one by grace, and leaves the other in sin, according to Matthew 24:40: "Two shall be in a bed [Vulgate: 'field' ['Bed' is the reading of Luke 17:34], one shall be taken, and one shall be left." Therefore respect of persons is not a sin.
On the contrary, Nothing but sin is forbidden in the Divine law. Now respect of persons is forbidden, Deuteronomy 1:17: "Neither shall you respect any man's person." Therefore respect of persons is a sin.
I answer that, Respect of persons is opposed to distributive justice. For the equality of distributive justice consists in allotting various things to various persons in proportion to their personal dignity. Accordingly, if one considers that personal property by reason of which the thing allotted to a particular person is due to him, this is respect not of the person but of the cause. Hence a gloss on Ephesians 6:9, "There is no respect of persons with God [Vulgate: 'Him']," says that "a just judge regards causes, not persons." For instance if you promote a man to a professorship on account of his having sufficient knowledge, you consider the due cause, not the person; but if, in conferring something on someone, you consider in him not the fact that what you give him is proportionate or due to him, but the fact that he is this particular man (e.g. Peter or Martin), then there is respect of the person, since you give him something not for some cause that renders him worthy of it, but simply because he is this person. And any circumstance that does not amount to a reason why this man be worthy of this gift, is to be referred to his person: for instance if a man promote someone to a prelacy or a professorship, because he is rich or because he is a relative of his, it is respect of persons. It may happen, however, that a circumstance of person makes a man worthy as regards one thing, but not as regards another: thus consanguinity makes a man worthy to be appointed heir to an estate, but not to be chosen for a position of ecclesiastical authority: wherefore consideration of the same circumstance of person will amount to respect of persons in one matter and not in another. It follows, accordingly, that respect of persons is opposed to distributive justice in that it fails to observe due proportion. Now nothing but sin is opposed to virtue: and therefore respect of persons is a sin.
Reply to Objection 1. In distributive justice we consider those circumstances of a person which result in dignity or right, whereas in respect of persons we consider circumstances that do not so result.
Reply to Objection 2. Persons are rendered proportionate to and worthy of things which are distributed among them, by reason of certain things pertaining to circumstances of person, wherefore such conditions ought to be considered as the proper cause. But when we consider the persons themselves, that which is not a cause is considered as though it were; and so it is clear that although persons are more worthy, absolutely speaking, yet they are not more worthy in this regard.
Reply to Objection 3. There is a twofold giving. one belongs to justice, and occurs when we give a man his due: in such like givings respect of persons takes place. The other giving belongs to liberality, when one gives gratis that which is not a man's due: such is the bestowal of the gifts of grace, whereby sinners are chosen by God. On such a giving there is no place for respect of persons, because anyone may, without injustice, give of his own as much as he will, and to whom he will, according to Matthew 20:14-15, "Is it not lawful for me to do what I will?... Take what is thine, and go thy way."
Source: Summa Theologica (New Advent)