Does God not know our sins already?
We are born into a world of sin and death.
Because Adam disobeyed God, he brought sin band death upon himself and his descendants.
We all sin and we all die.
Romans 5:12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
God has provided a saviour, Jesus Christ, who lived a sinless life and gave it in sacrifice that we might have our sins forgiven.
To avail ourselves of this provision we must believe and obey the gospel preached by Jesus and his apostles.
Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature. 16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned (condemned).
When we understand the gospel we must repent of our sinful way of life and be baptised by immersion in water for the remission of our past sins.
Acts 2:38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins…
Acts 22:16 And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
After baptism we are to live a new life by the commandments of Jesus Christ.
Romans 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
John 15:13 Greater love hath no man than this, that a man lay down his life for his friends. 14 Ye are my friends, if ye do whatsoever I command you.
Matthew 28:20 Teaching them to observe all things whatsoever I have commanded you:
We are instructed to be holy.
1 Peter 1:15 But as he which hath called you is holy, so be ye holy in all manner of conversation; 16 Because it is written, Be ye holy; for I am holy.
In practice we cannot live a sinless life.
If we could, Jesus need not have died for us.
After baptism, despite our best efforts, we will still sin.
John tells us that if we want forgiveness for these sins we must confess them in prayer.
1 John 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
From this we see that after baptism we need to confess our sins if we want forgiveness for them.
I hope you find this helpful.
If you would like to respond to, or have questions arising from this answer, please email me direct at firstname.lastname@example.org