Adam Clarke on 1 John 5:10

He that believeth on the Son of God - This is God's witness to a truth, the most important and interesting to mankind. God has witnessed that whosoever believeth on his Son shall be saved, and have everlasting life; and shall have the witness of it in himself, the Spirit bearing witness with his spirit that he is a child of God. To know, to feel his sin forgiven, to have the testimony of this in the heart from the Holy Spirit himself, is the privilege of every true believer in Christ.