That depends on the reason you would be leaving. If the pastor has wronged you, definitely tell him in person and confront him. If deacons are pedophiles as many are and have been for generations, tell the police first. If the preacher is preaching hatred, as 'hate homosexuals' meet him in the pulpit at the end of a sermon and tell everybody present as you tell him. If your beliefs are different, should you tell him in person, you will get a lecture and lesson and preached at. Therefore, write simply 'take my name off membership because I no longer believe the same as this church. I did that except, as a Baptist there, I took the Preamble, list of beliefs always in the songbook, and rewrote them to indicate how I had begun to believe differently.