Objection 1. It would seem that charity demands of a man to show his enemy the signs or effects of love. For it is written (1 John 3:18): "Let us not love in word nor in tongue, but in deed and in truth." Now a man loves in deed by showing the one he loves signs and effects of love. Therefore charity requires that a man show his enemies such signs and effects of love.
Objection 2. Further, Our Lord said in the same breath (Matthew 5:44): "Love your enemies," and, "Do good to them that hate you." Now charity demands that we love our enemies. Therefore it demands also that we should "do good to them."
Objection 3. Further, not only God but also our neighbor is the object of charity. Now Gregory says in a homily for Pentecost (In Evang. xxx), that "love of God cannot be idle for wherever it is it does great things, and if it ceases to work, it is no longer love." Hence charity towards our neighbor cannot be without producing works. But charity requires us to love our neighbor without exception, though he be an enemy. Therefore charity requires us to show the signs and effects of love towards our enemies.
On the contrary, A gloss on Matthew 5:44, "Do good to them that hate you," says: "To do good to one's enemies is the height of perfection" [Augustine, Enchiridion lxxiii]. Now charity does not require us to do that which belongs to its perfection. Therefore charity does not require us to show the signs and effects of love to our enemies.
I answer that, The effects and signs of charity are the result of inward love, and are in proportion with it. Now it is absolutely necessary, for the fulfilment of the precept, that we should inwardly love our enemies in general, but not individually, except as regards the mind being prepared to do so, as explained above (Article 8).
We must accordingly apply this to the showing of the effects and signs of love. For some of the signs and favors of love are shown to our neighbors in general, as when we pray for all the faithful, or for a whole people, or when anyone bestows a favor on a whole community: and the fulfilment of the precept requires that we should show such like favors or signs of love towards our enemies. For if we did not so, it would be a proof of vengeful spite, and contrary to what is written (Leviticus 19:18): "Seek not revenge, nor be mindful of the injury of thy citizens."
But there are other favors or signs of love, which one shows to certain persons in particular: and it is not necessary for salvation that we show our enemies such like favors and signs of love, except as regards being ready in our minds, for instance to come to their assistance in a case of urgency, according to Proverbs 25:21: "If thy enemy be hungry, give him to eat; if he thirst, give him... drink." Outside cases of urgency, to show such like favors to an enemy belongs to the perfection of charity, whereby we not only beware, as in duty bound, of being overcome by evil, but also wish to overcome evil by good [Romans 12:21, which belongs to perfection: for then we not only beware of being drawn into hatred on account of the hurt done to us, but purpose to induce our enemy to love us on account of our kindliness.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)