Objection 1. It would seem that it is lawful to communicate with unbelievers. For the Apostle says (1 Corinthians 10:27): "If any of them that believe not, invite you, and you be willing to go, eat of anything that is set before you." And Chrysostom says (Hom. xxv super Epist. ad Heb.): "If you wish to go to dine with pagans, we permit it without any reservation." Now to sit at table with anyone is to communicate with him. Therefore it is lawful to communicate with unbelievers.
Objection 2. Further, the Apostle says (1 Corinthians 5:12): "What have I to do to judge them that are without?" Now unbelievers are without. When, therefore, the Church forbids the faithful to communicate with certain people, it seems that they ought not to be forbidden to communicate with unbelievers.
Objection 3. Further, a master cannot employ his servant, unless he communicate with him, at least by word, since the master moves his servant by command. Now Christians can have unbelievers, either Jews, or pagans, or Saracens, for servants. Therefore they can lawfully communicate with them.
On the contrary, It is written (Deuteronomy 7:2-3): "Thou shalt make no league with them, nor show mercy to them; neither shalt thou make marriages with them": and a gloss on Leviticus 15:19, "The woman who at the return of the month," etc. says: "It is so necessary to shun idolatry, that we should not come in touch with idolaters or their disciples, nor have any dealings with them."
I answer that, Communication with a particular person is forbidden to the faithful, in two ways: first, as a punishment of the person with whom they are forbidden to communicate; secondly, for the safety of those who are forbidden to communicate with others. Both motives can be gathered from the Apostle's words (1 Corinthians 5:6). For after he had pronounced sentence of excommunication, he adds as his reason: "Know you not that a little leaven corrupts the whole lump?" and afterwards he adds the reason on the part of the punishment inflicted by the sentence of the Church when he says (1 Corinthians 5:12): "Do not you judge them that are within?"
Accordingly, in the first way the Church does not forbid the faithful to communicate with unbelievers, who have not in any way received the Christian faith, viz. with pagans and Jews, because she has not the right to exercise spiritual judgment over them, but only temporal judgment, in the case when, while dwelling among Christians they are guilty of some misdemeanor, and are condemned by the faithful to some temporal punishment. On the other hand, in this way, i.e. as a punishment, the Church forbids the faithful to communicate with those unbelievers who have forsaken the faith they once received, either by corrupting the faith, as heretics, or by entirely renouncing the faith, as apostates, because the Church pronounces sentence of excommunication on both.
With regard to the second way, it seems that one ought to distinguish according to the various conditions of persons, circumstances and time. For some are firm in the faith; and so it is to be hoped that their communicating with unbelievers will lead to the conversion of the latter rather than to the aversion of the faithful from the faith. These are not to be forbidden to communicate with unbelievers who have not received the faith, such as pagans or Jews, especially if there be some urgent necessity for so doing. But in the case of simple people and those who are weak in the faith, whose perversion is to be feared as a probable result, they should be forbidden to communicate with unbelievers, and especially to be on very familiar terms with them, or to communicate with them without necessity.
This suffices for the Reply to the First Objection.
Reply to Objection 2. The Church does not exercise judgment against unbelievers in the point of inflicting spiritual punishment on them: but she does exercise judgment over some of them in the matter of temporal punishment. It is under this head that sometimes the Church, for certain special sins, withdraws the faithful from communication with certain unbelievers.
Reply to Objection 3. There is more probability that a servant who is ruled by his master's commands, will be converted to the faith of his master who is a believer, than if the case were the reverse: and so the faithful are not forbidden to have unbelieving servants. If, however, the master were in danger, through communicating with such a servant, he should send him away, according to Our Lord's command (Matthew 18:8): "If... thy foot scandalize thee, cut it off, and cast it from thee."
With regard to the argument in the contrary [The Leonine Edition gives this solution before the Reply to Objection 2 sense the reply is that the Lord gave this command in reference to those nations into whose territory the Jews were about to enter. For the latter were inclined to idolatry, so that it was to be feared lest, through frequent dealings with those nations, they should be estranged from the faith: hence the text goes on (Deuteronomy 7:4): "For she will turn away thy son from following Me."
Source: Summa Theologica (New Advent)