Because unsaved people are spiritually dead, they can never be saved by self-reformation, regardless of how hard they try. Unsaved people need a new nature, not just new outward actions. You can take a pig, wash it up nice and clean, spray some perfume on it, and tie a pink ribbon around its neck, but all you will have is a cleaned-up pig! His nature will still be the same. And it won’t be long before he smells foul and is lying in mud again.
The same is true concerning religious people who have never been born again. They may be cleaned up on the outside a little bit, but inside they are as dirty as ever. Jesus said to some very religious people of His day,
Woe to you, scribes and Pharisees, hypocrites! For you clean the outside of the cup and of the dish, but inside you are full of robbery and self-indulgence. You blind Pharisee, first clean the inside of the cup and of the dish, so that the outside of it may become clean also. Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs which on the outside appear beautiful, but inside they are full of dead man’s bones and all uncleanness. So you, too, outwardly appear righteous to men, but inwardly you are full of hypocrisy and lawlessness (Matt. 23:25-28).
Jesus’ words are an apt description of all those who are religious but who have never experienced the new birth of the Holy Spirit. The new birth cleans up people on the inside and not just on the outside.