Sure, you can put whatever you want to put on your car. That's why they have bumper stickers for sale.
Or they can go through the same process the people who got the "In Reason" plates did. They're just trying to make a statement with it, instead of just doing it. They could have already had it done if they went the way of the In Reason plates. Then everyone is happy.
I find this pretty sad. I actually have an "I believe" license plate (yes, I am certainly an atheist). My father wound up getting it for some reason or another, and I really don't care about the plate one way or another. The way I see it, it's a sheet of metal with paint on it, and it in no way makes any statement about whether or not there is a god.
So when you see a black escort with the "I believe" license on it, don't assume that they actually do ;)
With that said, I really don't understand why the religious feel persecuted in some way that they have to christianize everything. Are they going to have priests bless the plates as they are made? I just don't get what the big deal is about a license plate.
The only problem I can think of is combining church and state. I don't see how the establishment clause of the constitution applies here, as congress is not approving these plates, unless the state constitution has a similar clause (I'd be happy to see that it does, but somehow I really don't expect it does).
I think the need to turn everything into a religious statement just shows how weak faith is for some, that if faith can be removed from one thing, it can be removed from all. As I said, I don't care if they have their plate (but we should be able to have ours!), and I actually have an "I believe" plate on my car as an atheist. Call it irony if you want.
"With that said, I really don't understand why the religious feel persecuted in some way that they have to Christianize everything."
I have no problem with Christians proselytizing as long as the same freedom exists in opposing groups including atheists. I'm sick and tired of having to "bite my tongue" when someone starts preaching a bunch of crap. Ridiculing is not persecuting, it neither maltreats or oppresses the rights of another.
Freedom of speech should, and does, include the freedom to ridicule the speech of others.