I haven’t noticed this stuff because 1) I don’t see any billboards where I live—they may be illegal here, 2) I never watch TV, 3) I don’t read newspapers, 4) about the only radio station I listen to is Christian, commercial-free, and listener-supported; although there are what could be called quasi-commercials on it. About all I remember of Christian advertising is the “Jesus Saves” billboards I used to see years ago in the South.
If you have billboards at all, it's not illegal for Christian groups to advertise on them. It would be a violation of first amendment rights to prohibit religious expression. I can guarantee you it's not illegal where you are.