We are planning on doing the same as you, raising the kid spiritually, but not under any organized religion. I was raised Lutheran. He is Jewish on his Mom's side and his dad is Japanese so he is kinda shinto, kinda christian, not really though. We have it a little easier than you I guess because nobody is gonna really care about it either way. My grandparents would have cared, but they already left the earth and don't get to meet the little one in the flesh. My mom is even exploring eastern religions and philosophies a lot on her own.
Anyway, I think your best weapon will be education in helping them all to understand why you feel the way you do. Maybe your parents don't know a lot about other religions and are Christian because that is how they were brought up, and had no reason to ever doubt it. Tell them about all the good points and bad points of the other religions that are out there... especially including why you think other religions have some good ideas and why Christianity has some bad ideas. Then assure them that you wanna teach good morals to your kids and it's not that you didn't learn anything from your Christian upbringing, but you just don't beleive in everything they have to say. I hope you'll be able to make them understand and accept it.
I think it is way more important to raise kids knowing right from wrong, and how to love and appreciate people and things and be a good person than what religion they are raised. Plenty of people are devout to their religion, but they can still be pretty horrible folks, so what it the point?
Make them listen to you. Good luck!