Use their first name, frequently. It is one of the best ways of bringing a very emotional person back to their basic values by reminding them (and you) who they really are - not an Angry Spokesperson for All Atheists, not an Embodiment of a Hated Philosophical Stance, but Jerry your Co-Worker, with whom you normally get along pretty well.
Be likable. Be as charming as you can, while holding fast to your views. It’s hard to be angry or hostile against someone who seems nice and kind.
Don’t tell them to “calm down.” In any confrontation, this doesn’t help, as it implies that he is the one who is out of control. Even if he is, it will probably be considered insulting and make the situation worse,
Don’t insult their beliefs, however offensive they may be to you personally. They think they have good and rational reasons for believing in what they believe.
Focus on shared points of agreement. We all like praise, even from someone with whom we disagree. If their hostility towards you or your beliefs derives from a feeling that you don’t take them seriously, or that you think they are sinful or a bad person, try acknowledging what is worthwhile in their arguments. “You bring up an interesting point…” “The fact that you’re saying that shows that you place a high value on human life and dignity…” “A lot of what you say is not that different from what we as Catholics believe…”, then use that shared agreement as a base for discussing what you believe.
When someone goes off on a rant, a good way to try to bring them back to a reasonable discourse is to ask, “I see you feel very strongly about this issue. Can I ask what personally led you to your views?” Behind many philosophical stances lies a deep personal hurt. Exploring that may help you understand why they feel they way they do, and give you the basis for correcting any erroneous beliefs.
We all want to have past wrongs acknowledged. If their beliefs derive, at least in part, from an action by Catholics (or Christians in general) in the past (“You people are all hypocrites. When I needed help the most, all you church-goers turned your back on me.”) apologize on behalf of whichever group they feel wronged by. "(“I’m sorry that happened to you. I like to think that I and the people I go to Mass with wouldn’t have treated you that way. For what it’s worth, I apologize on behalf of my people.”) In meeting an old emotional hurt, you will be surprised at how effectively this can reduce tension.
Ask them to define any terms they are using poorly or lazily, such as “creationism,” or “rational,” “science.” They are often arguing not against your beliefs, but a cartoon caricature of what they think your beliefs are.
Recognize when beliefs derive from politics (which are inherently emotional) instead of, or in addition to, philosophy. If someone begins arguing that the “Christian Right” is conspiring with George Bush to end all civil rights for gays, tell them what the Catholic Catechism actually does teach, and it can be helpful to point out how the Church was in the forefront of political movements with which they do, most likely agree.
Set boundaries on what is appropriate. If their language is hostile, abusive or profane, tell them that you can’t hold a conversation with them if their language is inappropriate, and see if that reduces the level if hostility. If not, tell them you should continue the conversation when they aren’t so upset.
Accept that you will probably not convert them in a single conversation, if at all. And no one ever changed their opinion as the result of an on-line argument. At best, you can show them that some of their arguments may be based on misconceptions about what Catholics believe, and give them some food for thought that could lead to a change in view.
