1 Peter 1:13-16

13 So think clearly and exercise self-control. Look forward to the special blessings that will come to you at the return of Jesus Christ. 14 Obey God because you are his children. Don’t slip back into your old ways of doing evil; you didn’t know any better then. 15 But now you must be holy in everything you do, just as God—who chose you to be his children—is holy. 16 For he himself has said, “You must be holy because I am holy.”

New Living Translation

This is really pretty clear. God is holy. He is too holy to be in the presence of sin. His very presence consumes impurity, like a fire. So when we are made God's children, and brought into His presence, He cleanses us. And we are to do our best to stay clean.

It is so easy to just accept the forgiveness He offers, and not make an attempt to life a holy life. But that isn't the way it is supposed to work. Peter is telling us not to slip back into our old habits, attitudes, or lifestyle.

We may not have understood before just how sinful and destructive those things were. But now we know, because we have seen that they were serious enough for Christ to have to suffer and die for them. God doesn't take sin lightly. Neither should we.