I’m of the view that these days we need to be direct with our words regarding what we believe. To paraphrase Selwyn’s words: people need to know that good, upright and moral living (alone) does not make a person one of Jesus’ followers.
The first and essential mark of a follower is that they believe Jesus is God and that He suffered and died for their sins on the cross.
The prayer which Selwyn uses is one, all true followers must say, in some form or other, and is worth repeating: “O God my Father, I come to you through Christ Your Son. I believe that He was sent from heaven to be my Saviour and I receive Him now into my heart by faith. Make me a true Christian. In Jesus’ name I pray. Amen.”