When I am tempted to beat myself up over some past sin, I gratefully remember God's great love and forgiveness.
Psalm 103:8-14 (KJV)
The LORD is merciful and gracious, slow to anger, and plenteous in mercy.
He will not always chide: neither will he keep his anger for ever.
He hath not dealt with us after our sins; nor rewarded us according to our iniquities.
For as the heaven is high above the earth, so great is his mercy toward them that fear him.
As far as the east is from the west, so far hath he removed our transgressions from us.
Like as a father pitieth his children, so the LORD pitieth them that fear him.
For he knoweth our frame; he remembereth that we are dust.
For those who fear God, He has great mercy (verse 11). (To fear God means to reverence God or to have awe for Him, but it also means literally to fear God, if we think we can live in habitual sin!) He has removed our sin from us as far as east is from the west (verse 12). He pities us and knows that our human frame is weak (verses 13 and 14).
Although the phrase "sea of forgetfulness" does not appear in the Bible, the concept is.
Hebrews 8:12 (KJV)
For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
When God forgives sin He no longer remembers it.
Micah 7:19 (KJV)
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.
And God does cast all our sin into the depths of the sea.
This may raise a question for some. If God no longer remembers our sins, how can we be judged at the end of the age?
Believers are "judged" when they come to saving faith and are born again.
The only other judgment that pertains to believers is the "judgment seat of Yeshua." We read about this in Romans and 2 Corinthians.
Romans 14:10 (KJV)
But why dost thou judge thy brother? or why dost thou set at nought thy brother? for we shall all stand before the judgment seat of Christ.
2 Corinthians 5:10 (KJV)
For we must all appear before the judgment seat of Christ; that every one may receive the things done in his body, according to that he hath done, whether it be good or bad.
This judgment is not about whether or not the believer is saved, but about reward and loss. Once saved, forgiven sins will never haunt us again. God will not bring them up. It is the good and wonderful things that we have done that God will review. If there are many, our rewards will be great, but if there are few, we won't receive as many.
1 Corinthians 3:11-15 (KJV)
For other foundation can no man lay than that is laid, which is Jesus Christ.
Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble;
Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is.
If any man's work abide which he hath built thereupon, he shall receive a reward.
If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
So, dear believer, if you are saved and have trusted in Yeshua as your righteousness, don't dwell on those past sins. They are gone forever! They truly have been cast into the Sea of Forgetfulness!