I'm going to back out of an agreement to go to church
Why are you going to back out on going to church?
Because I don't want to spend my Sunday listening to a sermon
You agreed to go to church, so whether you are interested in the preacher's sermon or not, backing out on your promise is irresponsible. Follow through on your promise to attend church no matter what.
I don't want to sit through the Sermon. I promised my friend I would go but I'm going to break my promise.
You promised your friend you were going to go. If you didn't want to go, you shouldn't have made the promise.