A Catholic Must Have No Relations with Heretics
In a country like the United Sates with a Protestant majority, it is useful to know the true Catholic doctrine on how a Catholic should avoid relations with them unless such relations deal with religious topics with the aim of converting them.
St. Thomas Aquinas, with his customary clarity and solid argumentation, is today more timely than ever in order to combat the false Ecumenism that invaded the Catholic Church principally after Vatican II.
St. Thomas Aquinas
For two reasons one must not maintain relations with heretics. First, because of their excommunication since one must not have relations with excommunicated persons. Second, because of their heresy, and this for three reasons:
First, because of the danger that our relations may come to corrupt others, according to what is taught in the First Epistle to the Corinthians: ‘Evil communications corrupt good manners.’ (15: 33).
Second, to not appear that one gives any approval to their perverse doctrines. In the second Epistle of St. John it is said: ‘If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you. For he that saith unto him: God speed you, communicateth with his wicked works’ (1:10-11). Regarding this verse, the Gloss comments: ‘To talk with someone reveals communion, unless the talk is a duplicity, which should not occur among Catholics.’
Third, in order that our familiarity [with heretics] does not provide occasion to err for others. Another Gloss comments on this passage of St. John: “Even if you are not taken in by the heretic, seeing your familiarity with him, others can be fooled imagining that you take pleasure from such relations and believe in him.’ Still a third Gloss adds: ‘The Apostles and Disciples exercised such great vigilance in religious matters that they did not even have an exchange of words with those who had removed themselves from the truth.’
The exception to this rule is when someone speaks about salvation with a heretic with the intention to save him.
(St. Thomas Aquinas, Quaestionis quodlibetales
[spontaneous questions], quodlibeto 10, q. 7, a. 1, 15, c.)