I frequently see people driving around in cars, trucks, SUVs, etc. who want me to know about their strong Christian faith. Their bumper stickers proclaim it, as do their little Christian fish symbols. I even saw a guy this weekend who felt compelled to make a giant fish symbol out of reflective tape for his door.
It's fine with me. It's your car, so go ahead and use it to spread whatever message you like.
But understand that the government can't help you. Thus, putting a cross on a state license plate goes too far.