Jesus answered and said unto them, This is the work of God, that ye believe on Him whom He hath sent. – John 6:29

Awakened souls, though they may have heard the gospel, do not in their hearts understand it. Many enquirers do not know what faith is. I am persuaded millions of our fellow country-men do not know what believing in Jesus means. Though every Sabbath-day they are told, yet do they not catch the thought, for the Spirit of God has not illuminated their minds. To believe in Jesus, as we say again, and again, and again, is simply to trust in Jesus-to take God at His word, to take Christ for what God says He is, namely, the atonement, the satisfaction for sin, the Savior of sinners…Many, though desirous to be saved, do not understand the work of Christ, or know what atonement is. Though the doctrine of substitution, which is the very marrow of the gospel, is to believers so very plain, yet many seekers have not learned it. That Jesus bore the sin of His people; that “the Lord hath laid on Him the iniquity of us all;” that He was made a sin for us; that justice received its due at His hands; this precious fact many penitent sinners have not grasped. They still think there is so much repentance to do, so much feeling to endure, so much praying to go through, so much mystery to be experienced; but the plain, simple precept, Believe and live; trust and be accepted, hide under the shadow of the cross and be safe-this, through ignorance, they do not understand, and this involves them in trouble upon trouble, till their way is hedged up with thorns…How soon would their bewilderment end in sweet repose, if they would obey the divine mandate and accept the great salvation! ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0947.cfm