How Does God Look at Your Sin?

We all sin and fall short of the glory of God. But we can repent and ask for forgiveness, and because of Christ’s Sacrifice, we CAN be forgiven. When you see a brother or sister struggle with sin, you can and should pray to God that He would help him or her, and give him or her strength to overcome and to continue. However, what did John mean when he said that we should not pray for those who commit sin “leading to death”? What is this sin–and is there nothing we can do in such a case?