He will turn again, He will have compassion upon us; He will subdue our iniquities; and Thou wilt cast all their sins into the depths of the sea. – Micah 7:19

When a sinner believes in Christ, his sins positively cease to be, and what is more wonderful they all cease to be…They are all swept away in one solitary instant; the crimes of many years; extortions, adulteries, or even murder, wiped away in an instant; for you will notice the absolution was instantaneously given. God did not say to the man-“Now you must go and perform some good works, and then I will give you absolution.” He did not say as the Pope does, “Now you must swelter awhile in the fires of Purgatory, and then I will let you out.” No, he justified him there and then; the pardon was given as soon as the sin was confessed. “Go, my son, in peace; I have not a charge against thee; thou art a sinner in thine own estimation, but thou art none in mine; I have taken all thy sins away, and cast them into the depth of the sea, and they shall be mentioned against thee no more for ever.”

The pardon which is sealed in heaven is re-sealed in our own conscience. The mercy which is recorded above is made to shed its light into the darkness of our hearts. Yes, a man may know on earth that his sins are forgiven, and may be as sure that he is a pardoned man as he is of his own existence. And now I hear a cry from some one saying, “And may I be pardoned this morning? and may I know that I am pardoned? May I be so pardoned that all shall be forgotten-I who have been a drunkard, a swearer, or what not? May I have all my transgressions washed away? May I be made sure of heaven, and all that in a moment?” Yes, my friend, If thou believest in the Lord Jesus Christ, if thou wilt stand where thou art, and just breathe this prayer out, “Lord, have mercy! God be merciful to me a sinner, through the blood of Christ.” I tell thee man, God never did deny that prayer yet; if it came out of honest lips He never shut the gates of mercy on it. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0216.cfm