He who believes in the Son of God has the witness in himself… These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life and that you may continue to believe in the name of the Son of God.- 1 John 5:10, 13

Faith accepts the witness of God, and more, He that hath faith has the witness in himself to the truth of God. “He that believeth on the Son of God hath the witness in himself.” It is not said, “He that does this or feels that,” but “He that believeth hath the witness in himself,” his heart bears witness to the truth of God. Has any unsaved man an experimental witness within? Will you tell me that a man’s inner experience bears witness to God’s gospel and yet the man is in a lost state, or only hopeful of being saved ultimately? No, sir, it is impossible. He that believeth has that change wrought in him which enables his own consciousness to confirm the witness of God, and such a man must be in a state of salvation. It is not possible to say of him that he is an unsaved man.

Our Lord Himself, and His apostles, in several places have declared, “He that believeth on Him hath everlasting life.” Do not tell me that a sinner who believes in Jesus is to make an advance before he can say he is saved, that a man who trusts Christ is only on his way to salvation, and must wait until he has used the ordinances, and has grown in grace, before he may know that he is saved. No, the moment that the sinner’s trust is placed on the finished work of Jesus he is saved. Heaven and earth may pass away, but that man shall never perish. If only one second ago I trusted the Saviour I am safe; just as safe as the man who has believed in Jesus fifty years, and who has all the while walked uprightly. I do not say that the new born convert is as happy, nor as useful, nor as holy, nor as ripe for heaven, but I do say that the words, “he that believeth on Him hath everlasting life,” is a truth with general bearings, and relates as much to the babe in faith as does to the man who has attained to fullness of stature in Jesus Christ. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0979.cfm