The bible is filled with paradoxes.
God knows that we've sinned even when it's "forgotten" because He's still God.
Yet when we come to Him in repentance, all our sin is wiped out and God never reminds us of it. No longer does our sin interfere with our salvation, praise God!
Micah 7:18-19
Where is another God like you,
who pardons the guilt of the remnant,
overlooking the sins of his special people?
You will not stay angry with your people forever,
because you delight in showing unfailing love.
19 Once again you will have compassion on us.
You will trample our sins under your feet
and throw them into the depths of the ocean!
Hebrews 8:12
And I will forgive their wickedness,
and I will never again remember their sins.”
Hebrews 10:17
Then he says,
“I will never again remember
their sins and lawless deeds.”