Colossians 3:9-10

Do not lie to one another, for you have stripped off the old self with its evil practices and have put on the new (spiritual) self who is being continually renewed in true knowledge in the image of Him who created the new self.

Paul is not talking to unsaved people here, but the Christians in the Colossian church. They were working through and learning about salvation and following Jesus as believers. We, as believers, need to continually renew ourselves in the word and deepen our knowledge of Jesus. Whether long-time believers or new, there is an enemy trying to steal our faith and causing us to sin.