I have an experience that *might* help you with this situation, but I'm not sure. Because the grilling I got wasn't from a formal organization, but from an elderly, well respected but cantankerous relative. Along with another relative who I dearly respected and loved, from technically the same church, and my aunt and my mother. See, he was the family lawyer, as well as my grandfather's brother. Because of the presence of the other relative, and my aunt and mother, I had to remain very very calm and very very polite. And I couldn't badmouth Christianity, because they were all Christian, and I didn't want to get summarily kicked out of the family. Also, luckily, I didn't have such a hate-on for the religion anymore, so it was easier in a sense for me in that small regard.
No, the hardest part was that my inquisitor had no obligation to be kind, and I did.
The best advice I have for you is to pretend in your heart that this is a debate. Yes, they will defame your character, your morals and damn your soul in front of everybody. Prepare for the worst, but don't dish it back at them no matter how tempting it is. Trust me on this one.
Even if you don't have a proper label for your path at this time, clarify to yourself what your current beliefs are. Write them down if necessary. Know what they are so you can use them in your discussion. That way, no matter what they say, you present a unified polite front that cannot be assailed.
Because no matter how unified they may seem, there will always be someone who wonders about the other side, and that, my friend, will be you. However, there's something else you need to do. Mark everything they *don't* do that's negative. Keep those things in mind, mark them, and remember them. Because throughout this ordeal, you must try to respect them at some level for some thing. That way you can still keep an even temper about the whole thing. No pressure, but you don't only represent yourself, but all people of alternative religions to them. Do not let them goad you into saying things you don't mean, or things you don't want to say in front of a judge and jury.