The next time you're faced with one of the really angry Cult of Gnu sorts - the kind who rail against Christians as monsters who are mentally ill and need to be re-educated by the state, or bullied or taunted or mocked into silence and submission, ask them this simple question.
"So if your daughter told you she accepted Christ, how hard would you hit her?"
I won't lie: ask this question, and you're playing with fire. It will provoke outrage, and quite possibly a ban on most forums. You don't drop this in conversation with someone who is civil, or for whom civil conversation remains as a possibility. But you do drop it against the pathological, the people who just positively hate Christians, and religious believers in general. Because, for all the 'How DARE you, what a HORRIBLE question' shit they'll lash out with, the reason for the lashing will be simple: because it's an effective question which threatens to cripple the emotional high of the hatred they rely on. It opens too many intellectual scars, it lays down too many emotional landmines, and it generally forces them to engage in a kind of self-reflection they both sorely need, yet subconsciously avoid like crazy.
Note that this won't accomplish much with all of this sort. Really, you're dealing with fringe-cases here who, like Madelyn Murray O'Hare, may well feel quite comfortable with utterly disowning any child who became Christian, in theory. But they'll still be generally socially cognizant enough to know that saying as much would make them look like shit, and they will resent the restraint you've forced them to show in their reply.