Can Christians sin? Will God forgive them (or us), even if we sin again and then some? Of course. Just as importantly, if you see a believer sinning, then you should pray for them:
1 John 5:16-17 If you see a fellow believer sinning in a way that does not lead to death, you should pray, and God will give that person life. But there is a sin that leads to death, and I am not saying you should pray for those who commit it. All wicked actions are sin, but not every sin leads to death.
The sin that leads to death is that ever-so-famous unforgivable sin—the sin of not caring anymore about God, conscience or sinning and never caring again. (If you’re worried you’ve committed that sin, then it means you haven’t. Obviously, you still care, or you wouldn’t be worried!)
All we have to do is ask forgiveness:
1 John 1:8-10 If we claim we have no sin, we are only fooling ourselves and not living in the truth. But if we confess our sins to him, he is faithful and just to forgive us our sins and to cleanse us from all wickedness. If we claim we have not sinned, we are calling God a liar and showing that his word has no place in our hearts.
Jesus is our intercessor, and He has already dealt with our sins:
1 John 2:1-2 My dear children, I am writing this to you so that you will not sin. But if anyone does sin, we have an advocate who pleads our case before the Father. He is Jesus Christ, the one who is truly righteous. He himself is the sacrifice that atones for our sins—and not only our sins but the sins of all the world.