OK, very good. (And I put the disclaimer in there so you didn't think I was being sarcastic or something ;o).
So to put it together, the purpose of the church is to spread the gospel, and the purpose of spreading the gospel is to save souls.
Now, once the souls are saved, what do you do with them then? Is there a point in continuing to preach a salvation message at every service to a group of people who are all already saved? Or, as Mack would say, should you move them from "milk" to "meat"?
He gives the plan of salvation every Sunday at the invitation. It doesn't have to be a big thing, just something that might cause someone who is seeking to ask questions as to what it is all about.
Becky