The sinner’s need for the gospel doesn’t end when he is converted. While the fullness of eternal life is bestowed upon the vilest sinner at that point, he still needs the good news to grow him, mature him, sanctify him.
I have a friend who argues that there should not be an element in the Lord’s Day worship gathering called “preaching.” He says that in the New Testament, the concept of preaching is almost exclusively bound up with evangelism — preaching the gospel — and thus what happens during the sermon time should be simply called teaching and conducted more along the lines of instruction than proclamation. Preaching, then, is what takes place outside the four walls of the church in our sharing of the good news with the lost.
I agree with his premise but not his conclusion. Yes, the concept of preaching in the New Testament is almost exclusively bound up with the proclamation of the gospel. But I believe my friend’s error is in believing that the gospel is only for the lost.
Christians need the gospel as much as lost people do. Not in the same way, of course. But just as much.