My 18 year old nephew, who was not raised with religion, is now a Christian and believes that since I don't believe in Jesus, I am going to hell. That was a shock to hear from him. Unfortunately, he and my niece have had a rough life and it's possible that he has found some sense of belonging, when he didn't really have that before. Still sad that he found a immoral, bigoted organization to grab onto.
The funny thing is that my family and I talked about baptizing my brother's kids when they were tiny, to save their poor heathen souls :( (We didn't, since we knew that type of action is personal and the final decision was not up to us).
I agree with the other posts here. In spite of the fact that you've probably played out possible conversations in your mind, you won't know their reactions or what you will say until you are having that conversation with your family. All you can really do is state how you will raise your child and why - just as you will do with your child when he/she is old enough. It's not as easy as stating you follow one political party and your family follows the other. It sounds like you are prepared for a confrontation and that obviously helps.
I raised my sons in the Roman Catholic faith and they lead me to atheism, because I taught them to think for themselves, explained what was right and wrong and why, and basically modeled the behavior and actions I wanted them to learn. Here is a GREAT book for secular parents:
Without knowing it, I followed a lot of the advice given in the book and raised Freethinker! Be confident in your decision and know that what you are doing is best for YOUR child. Stand firm and your child will be strong, too :)