If anyone see a brother sin not unto death, God will give them life  -  1 John 5:16

If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.

John gave an instance in which a believer can have confidence in prayer. If anyone sees their brother in Christ sinning a sin which doesn’t lead to death, they can ask, and God will give life to those who commit that type of sin. This is a case where a Christian sees a fellow believer engaging in some sinful activity.