You can’t resign from a church.......you aren’t an officer, just a member.
You can withdraw membership though.
Good point. Many churches will not grant a letter dimissory (or a letter of transfer) to a member without knowing where they’re headed — otherwise they are still considered members. I’m not sure what the UCC’s practice is.
Of course, ecclesiastically speaking, he’s just made himself a non-Christian, having severed his ties to Christ’s body.