God makes no promise that He will use my story as His power unto salvation. The gospel is not about me. The gospel is about Jesus. It is the proclamation of the person and work of Christ, and of how a person can appropriate the benefits of the work of Christ by faith alone.
We see this from our passage in John’s Gospel. The healed man could say, “I once was blind, but now I see,” and that was a wonderful testimony. But it was not the gospel. The man could not tell the Pharisees about Jesus’ saving work and about how they could be delivered from their sins by faith in Him. So we need to learn not only our testimonies but the concrete elements and content of the biblical gospel. Evangelism takes place when the evangel is proclaimed and announced to people—that is the gospel.
– “Your Testimony Is Not the Gospel” by RC Sproul