OP: They say we are already forgiven for our past, present, and future sins, and because of that our sins are overlooked. So there's no need to confess our sins, ask forgiveness, and repent.
What is the Biblical basis for this belief?
Couple of background points.
It is true our sins past, present, and future are forgiven in Christ.
But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness. Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works, Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered. Blessed is the man to whom the Lord will not impute sin. Romans 4:5-8
Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them. Heb 7:25
John's baptism was of repentance.
John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins. Mark 1:4
Repentance means "change of mind". What were they changing their mind about? The Mosaic Law's message that IF you do all of this LAW, THEN you will be forgiven (Deut 6:25). Recall that John was to make straight the path of the Lord (Isa 40:3).
Is the baptism of repentance for the Christian?
And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism. Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus. When they heard this, they were baptized in the name of the Lord Jesus. Acts 19:3-5
Change your mind, repent, of whatever else you think might "save" you, and be baptized in the name of Jesus Christ (or Trinity).
Now, to the question.
Once you are a believer, what about your sins?
We ask forgiveness, not because God is ready to send a thunderbolt your way, but because of your human relationship within the Body of Christ first, but also of those outside who we might offend.
Wherefore, if meat make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend. 1 Cor 8:13
So, to answer the OP, our sins are forgiven and forgotten (Psalm 103:12, Heb 10:10).
Still, we need to confess our sins and ask forgiveness and repent for the Body of CHrist.
PS. To be saved by grace through faith in Christ Jesus in the first place, we must acknowledge we are sinners (Rom 5:8, 1 Tim 1:5).