"Hi. Your church is in such a beautiful location for a bike ride. I've been using your parking lot for a while with no issues, as I only park here when there's no service going on. But I just wanted to see if there was someone I could talk to and make sure it was still OK to park here once or two evenings a week."
Most likely, the first person you talk to is going to say something like "I don't think it's a problem." If you do get pushed up the chain of command, always ask in a way that compliments the church or person, implies you have permission already, and forces them to be a jerk if they say no. "I hope it's OK if I keep parking at the far end of your lot once in a while. I've even taken a couple of selfies with this church as a backdrop."