To be Christian is to be one who has turned from sin and turned to God. Anything else may or may not be a Christian in name, may or may not be a person professing Christ as his Lord, but the new covenant is not a reprieve from the obligation for man to be righteous but is, in fact, the means to his obtaining that very thing. We love because He's loved us. And love we must, as John tells us in his first letter while also affirming the corresponding fact that sin must be overcome in a believer in order for them to even qualify as a true believer, as a child of God.
So a Christian, for example, forgives others out of love, reciprocating God's love and, "coincidentally", must do so (Matt 6:15). Now if that "Christian" becomes embroiled in sin all over again: the deeds of the flesh as outlined in Gal 5, "sin that leads to death" (1 John 5:16), having failed to keep his robes clean as per Rev 22:14, then he's effectively opposing love of God and neighbor and is destroying that love within himself. At that point he's outside of the fold and is no better off than a non-believer with or without any profession of faith-talk is quite cheap anyway. So what does he need to do? Certainly not just presume upon or take Gods forgiveness for granted; he must have a change of heart at some point and turn back away from sin-and to God again. Then he'll be rejoining the ranks of those who love, love being the authentic righteousness that man must be clothed in for God to recognize him as His own.
"If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness." 1 John 1:9
Christians don't remain sinners, not in the overt ways as the bible warns against. The gospel is about change-and not merely about the forgiveness of sin with no other change taking place in us but it's about the taking away of sin, of the sinful heart of stone that causes it and replacing it with a new heart. If I slap you in the face and you forgive me, but I keep doing it over and over, then nothing has changed in me-I'm just forgiven. But if I finally see and begin to value and embrace that love you've shown me and stop slapping you, then I've been affected by your love. We mock God and the work of His Son when we continue in sin or take His forgiveness for granted. (Matt 18:21-35). It's up to us whether or not we'll benefit from God's ever-present grace:
"Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life. " Gal 6:7-8