I John 5:16-17 If you see a Christian brother or sister 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. Every wrong is sin, but not all sin leads to death.
One of the ways you can know that you are truly connected to God in love is how you react to those around you when they sin against God. When we who are God's children sin, God does not reject us as unfit for His kingdom and cast us out. He graciously brings our sin up, reminds us of how awful sin is to Him, convicts us, leads us to repentance, forgives us, and leads us forward in love. We are to treat each other the same way. Instead of judging one another and treating each other harshly because of sin, we should be kind, gracious, and forgiving. We should never let the sin of a repentant heart keep us from embracing the sinner in love and having fellowship with Him. But, sadly we do. Instead of coming alongside of them in prayer, we cast them aside and treat them as unworthy of our fellowship. Shame on us.
There are times in which we have to sever fellowship because of unrepentant hearts. That is not the issue here. Here we are dealing with our heart attitude towards those who really do want to do what is right before God. We need to pray for them. If they don't recognize their sin, pray that the Lord will make it known to them so that they can repent. The goal always in Christ is restoration to a right relationship first with God and then others. Human relationships cannot be right until first our relationship with God is right.
What is the sin that leads to death? I believe it is a stubborn and willful rejection of God. It is an unwillingness to listen to God. It is a desire to have nothing to do with God. It is a heart that is spiteful towards God. Such a heart cannot hear God speak about sin and all sin leads to death of the soul.
We all sin, but we all are not bound for hell because of it. God forgives the sin of a repentant heart and judges the sin of the unrepentant heart. There is nothing that we can do when someone refuses to repent of sin. However, it is not for us to judge others. We are to continue to love them in spite of their sin and let God deal with them for their sin.
Is there someone you know who needs the love you have withheld from them because of their sin against you? It is a sin to not love those for whom Christ died. Confess your sin and ask the Lord to show and teach you how to love them.