Objection 1. It would seem that almsgiving is not a matter of precept. For the counsels are distinct from the precepts. Now almsgiving is a matter of counsel, according to Daniel 4:24: "Let my counsel be acceptable to the King; [Vulgate: 'to thee, and'] redeem thou thy sins with alms." Therefore almsgiving is not a matter of precept.
Objection 2. Further, it is lawful for everyone to use and to keep what is his own. Yet by keeping it he will not give alms. Therefore it is lawful not to give alms: and consequently almsgiving is not a matter of precept.
Objection 3. Further, whatever is a matter of precept binds the transgressor at some time or other under pain of mortal sin, because positive precepts are binding for some fixed time. Therefore, if almsgiving were a matter of precept, it would be possible to point to some fixed time when a man would commit a mortal sin unless he gave an alms. But it does not appear how this can be so, because it can always be deemed probable that the person in need can be relieved in some other way, and that what we would spend in almsgiving might be needful to ourselves either now or in some future time. Therefore it seems that almsgiving is not a matter of precept.
Objection 4. Further, every commandment is reducible to the precepts of the Decalogue. But these precepts contain no reference to almsgiving. Therefore almsgiving is not a matter of precept.
On the contrary, No man is punished eternally for omitting to do what is not a matter of precept. But some are punished eternally for omitting to give alms, as is clear from Matthew 25:41-43. Therefore almsgiving is a matter of precept.
I answer that, As love of our neighbor is a matter of precept, whatever is a necessary condition to the love of our neighbor is a matter of precept also. Now the love of our neighbor requires that not only should we be our neighbor's well-wishers, but also his well-doers, according to 1 John 3:18: "Let us not love in word, nor in tongue, but in deed, and in truth." And in order to be a person's well-wisher and well-doer, we ought to succor his needs: this is done by almsgiving. Therefore almsgiving is a matter of precept.
Since, however, precepts are about acts of virtue, it follows that all almsgiving must be a matter of precept, in so far as it is necessary to virtue, namely, in so far as it is demanded by right reason. Now right reason demands that we should take into consideration something on the part of the giver, and something on the part of the recipient. On the part of the giver, it must be noted that he should give of his surplus, according to Luke 11:41: "That which remaineth, give alms." This surplus is to be taken in reference not only to himself, so as to denote what is unnecessary to the individual, but also in reference to those of whom he has charge (in which case we have the expression "necessary to the person" [The official necessities of a person in position] taking the word "person" as expressive of dignity). Because each one must first of all look after himself and then after those over whom he has charge, and afterwards with what remains relieve the needs of others. Thus nature first, by its nutritive power, takes what it requires for the upkeep of one's own body, and afterwards yields the residue for the formation of another by the power of generation.
On the part of the recipient it is requisite that he should be in need, else there would be no reason for giving him alms: yet since it is not possible for one individual to relieve the needs of all, we are not bound to relieve all who are in need, but only those who could not be succored if we not did succor them. For in such cases the words of Ambrose apply, "Feed him that dies of hunger: if thou hast not fed him, thou hast slain him" [Cf. Canon Pasce, dist. lxxxvi, whence the words, as quoted, are taken]. Accordingly we are bound to give alms of our surplus, as also to give alms to one whose need is extreme: otherwise almsgiving, like any other greater good, is a matter of counsel.
Reply to Objection 1. Daniel spoke to a king who was not subject to God's Law, wherefore such things as were prescribed by the Law which he did not profess, had to be counselled to him. Or he may have been speaking in reference to a case in which almsgiving was not a matter of precept.
Reply to Objection 2. The temporal goods which God grants us, are ours as to the ownership, but as to the use of them, they belong not to us alone but also to such others as we are able to succor out of what we have over and above our needs. Hence Basil says [Hom. super Luc. xii, 18: "If you acknowledge them," viz. your temporal goods, "as coming from God, is He unjust because He apportions them unequally? Why are you rich while another is poor, unless it be that you may have the merit of a good stewardship, and he the reward of patience? It is the hungry man's bread that you withhold, the naked man's cloak that you have stored away, the shoe of the barefoot that you have left to rot, the money of the needy that you have buried underground: and so you injure as many as you might help." Ambrose expresses himself in the same way.
Reply to Objection 3. There is a time when we sin mortally if we omit to give alms; on the part of the recipient when we see that his need is evident and urgent, and that he is not likely to be succored otherwise--on the part of the giver, when he has superfluous goods, which he does not need for the time being, as far as he can judge with probability. Nor need he consider every case that may possibly occur in the future, for this would be to think about the morrow, which Our Lord forbade us to do (Matthew 6:34), but he should judge what is superfluous and what necessary, according as things probably and generally occur.
Reply to Objection 4. All succor given to our neighbor is reduced to the precept about honoring our parents. For thus does the Apostle interpret it (1 Timothy 4:8) where he says: "Dutifulness* [Douay: 'Godliness'] is profitable to all things, having promise of the life that now is, and of that which is to come," and he says this because the precept about honoring our parents contains the promise, "that thou mayest be longlived upon the land" (Exodus 20:12): and dutifulness comprises all kinds of almsgiving. ["Pietas," whence our English word "Piety." Cf. also inf, 101, 2.]
Source: Summa Theologica (New Advent)