It helps if you read that in context. Read John 5:1-15
John 5:15 "Afterward Jesus findeth him in the temple, and said unto him, Behold, thou art made whole: sin no more, lest a worse thing come unto thee."
Also John 8:1-11
John 8:10-11 "When Jesus had lifted up himself, and saw none but the woman, he said unto her, Woman, where are those thine accusers? hath no man condemned thee? She said, No man, Lord. And Jesus said unto her, Neither do I condemn thee: go, and sin no more."
In this instance he was telling her to stop sleeping around because the Pharisees were going to stone her.
If you aren't saved if you sin after getting saved, then no one is saved.