…he that believeth not the Son shall not see life; but the wrath of God abideth on him. – John 3:36

What is this peculiar sin which entails the wrath of God upon these people? It is that they have not believed the Son of God. What does that amount to? It amounts to this, first of all, that they refuse to accept the mercy of God. God made a law, and His creatures were bound to respect and obey it. We rejected it and turned aside from it. It was a great display of the heart’s hatred, but it was not in some respects so thoroughly and intensely wicked a manifestation of enmity to God as when we reject the Gospel of grace.

God has now presented not the law, but the Gospel to us. He has said, “My creatures, you have broken My law, you have acted very vilely towards Me. I must punish your sin, else I were not God, and I cannot lay aside My justice, but I have devised a way by which, without any injury to any of My attributes, I can have mercy upon you. I am ready to forgive the past, and to restore you to more than your lost position, so that you shall be My sons and My daughters. My only command to you is, believe in My Son. If this command be obeyed, all the blessings of My new covenant shall be yours. Trust Him and follow Him, for behold, I give Him as leader and commander to the people. Accept Him as making atonement by His substitution and obey Him.”

Now, to reject the law of God shows an evil heart of unbelief, but who shall say what a depth of rebellion must dwell in that heart which refuses not only the yoke of God, but even the gift of God? The provision of a Savior for lost men is the free gift of God, by it all our wants are supplied, all our evils are removed, peace on earth is secured to us, and glory forever with God—the rejection of this gift cannot be a small sin.

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/1012.cfm