“Wherefore, I say unto you, that ye ought to forgive one another; for he that forgiveth not his brother his trespasses standeth condemned before the Lord; for there remaineth in him the greater sin. I, the Lord, will forgive whom I will forgive, but of you it is required to forgive all men” (D&C 64:9).
“For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses” (Matthew 6:14-5).
“For that which ye do send out shall return unto you again, and be restored; therefore, the word restoration more fully condemneth the sinner, and justifieth him not at all” (Alma 41:15).
“Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap. For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting” (Galatians 6:7-8).
“For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember [mnaomai] no more” (Hebrews 8:12).
The Lord “remembers” a repented sin no more, but that does not mean that the sin is utterly erased from the Lord’s memory. In this context, remembering is more like recalling, or bringing back to our remembrance.
“And they shall teach no more every man his neighbour, and every man his brother, saying, Know the Lord: for they shall all know me, from the least of them unto the greatest of them, saith the Lord: for I will forgive their iniquity, and I will remember [zakar] their sin no more” (Jeremiah 31:34).
“If the wicked restore [shuwb] the pledge, give again that he had robbed, walk in the statutes of life, without committing iniquity; he shall surely live, he shall not die. None of his sins that he hath committed shall be mentioned [zakar] unto him” (Ezekiel 33:15-6).
It is not possible that the atonement and miracle of forgiveness alter history so that a repented sin never occurred. The innocent victims of terrible, abusive sins can forgive their abuser, but that does not change the reality of what happened.
It is not possible for us to know something that God does not know. Even if we repent of a sin, we can still have a memory of it. God has a perfect, unfaltering memory.
“The Lord hath sworn by the excellency of Jacob, Surely I will never forget any of their works” (Amos 8:7).
“O how great the holiness of our God! For he knoweth all things, and there is not anything save he knows it” (2 Nephi 9:20).
“I, the Lord, will not lay any sin to your charge; go your ways and sin no more; but unto that soul who sinneth shall the former sins return, saith the Lord your God” (D&C 82:7).
God would not be able to recall or bring previous sins back to our mind if He forgot them completely.