Back from traveling again. Thanks for your patience. I think the three conversational situations you describe need different responses. Let’s talk about them.
Concerning the first situation: When an anti-Christian acquaintance says something which is deliberately blasphemous, just explain that his statement offends you. If he continues, quietly say, “Let’s change the subject,” and begin a new one. If he persists, quietly say “I’m not willing to talk like this, so I’ll see you around,” and walk away. Don’t get angry, don’t apologize, and don’t back down. Don’t justify yourself, don’t stay to listen to his own self-justifications, and don’t explain. If your acquaintance is worth talking to, he will mend his conversational ways, and if he doesn’t, he isn’t.
Concerning the second situation: When a lapsed Catholic friend criticizes you for, say, not going with him to strip clubs, he knows perfectly well that going to strip clubs is wrong. He’s not criticizing because he thinks you’re mistaken, but because his own conscience is accusing him. Don’t tell him that, because it will merely make him defensive; just bear it in mind. Whenever he begins to razz you, say “You know why I don’t go to strip clubs, and I don’t need to justify myself. Should we change the subject, or end the conversation?” If he keeps at it, deal with him as with the friends I discussed above.
Concerning the third situation: When a Protestant friend makes unfounded and unreasonable claims against the Catholic faith, for example that Catholics worship devils or pray to idols, you need to discern two things. The first is his motive for speaking. Is he trying to rescue you from what he mistakenly considers your errors, or does he merely desire to insult your faith? The second thing to discern is whether he is willing to be corrected about what Catholics really believe. Putting these two things together, we have four possibilities.
(1) If the friend speaks with the first motive, is willing to be corrected, and is not argumentative, correct him. Your mode of correction should be to simply and briefly explain what Catholics actually believe.
(2) If he speaks with the first motive, is in principle willing to be corrected, but turns out to be too argumentative to be corrected effectively, tell him nicely that you don’t think the two of you are ready to have this conversation yet. Don’t end the matter there. You might suggest that he take a look at the appropriate section of the Catechism; he can find a convenient searchable version at www.scborromeo.org/ccc.htm . Or refer him to a reliable Catholic apologetics website, such as http://www.catholic.com/tracts . Perhaps conversation will become possible later.
(3) If he speaks with the first motive but is not willing to be corrected, don’t engage in discussion at all; change the subject.
(4) On the other hand, if he speaks with the second motive, speak to him as you would speak with the anti-Christian acquaintances who sling blasphemies.
It may be difficult to discern just which of these four possibilities is actual. One way to find out is to ask: “If you understood what Catholics believe, you’d understand that we aren’t really doing what you think we are doing. Let me ask you frankly: Are you interested enough and open-minded enough to listen to my explanation?”
Another way to find out is trial and error. If you find you were mistaken about a friend’s motive or open-mindedness, shift gears. Use common sense, because conversations are messier than I am describing them, and so are conversational histories. A friend may at first seem unable to speak with you reasonably, but as time goes on he may become more open-minded. Or a friend who at first seems reasonable may as time goes on become belligerent.
Use lots of charity and patience. It sounds like some of your Protestant friends are fundamentalists who respect you personally but have been taught things about Catholicism which are gravely mistaken. When I was young, I was taught some of those things too. If they were taught to your friend by persons whom he trusted, he may even suspect that you have been deceived about the teachings of your own faith!
Is this helpful? By the way, since many young Catholics find themselves in the same situations, I may use a version of this letter in my blog.