If anyone sees his brother sinning a sin which does not lead to death

1 John 5:16
If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death.

God through Jesus forgives the sins of Man, we they would come and confess their sins. God also leads others to pray for those who seek forgiveness, the ministry of intercession. So to be useful to God, by being intercessors, we ourselves must first be right with God. We too must come and confess and receive His forgiveness. In addition, to know the mind of Christ when He forgive others, there is no doubt that we too, must walk right with Yshua, and be in constant communion with Him. Only then are we sensitive to the spiritual needs of others and fulfill our role as intercessors.

and wondered that there was no intercessor

Isaiah 59:16
And he saw that there was no man; and wondered that there was no intercessor.

God was saying that he found no intercessor in this prayer. This may be a wake up call for us when we pray for each other. It is often heard in Christian circles that we pray for each other. But the kind of prayer that is effective when we are requesting God to help our fellow believers are intercessory prayers. Such intercessory prayers can be hard work, and we need to put our intense spiritual and physical focus on the prayer. The prayers have to be honest and we should pray almost to the point that we are looking at the situation from God's view when praying for others. We have to be close to God, understand what is wrong from God's point of view and what needs to change from God's point of view, in order to pray for change for our fellow believers. It is no surprise at all that the greatest intercessor is Jesus Himself, who being God, is also fully man, able to empathize with us as well as understand the sinfulness from God's view.