Now we know that John doesn't mean that Christians never sin because in v. A true Christian will not persist in unloving acts. A true Christian will not persist in unbelief. He or she may lapse temporarily into hard-heartedness. But the mark of a true Christian is that they will repent, confess their sin and be restored.
Sin is serious business.
Failing to trust God and refusing to love one another is serious business. The answer to this question leads us to the glorious truth in verse l6 that intercession for the spiritual welfare of a brother or sister has eternal significance. If anyone sees his brother committing what is not a mortal sin, he will ask and God will give him life for those whose sin is not mortal. There is sin which is not mortal. I do not say that one is to pray for that.