Then if we do not confess, does He forgive us our sins and to cleanse us from all unrighteousness?
For the unsaved person, no. God does not give forgiveness without asking or everyone would be saved. It was the people who came to Jesus whom He responded to.
For the saved person, if they sin after they're saved, they need to confess that sin to open the lines of communication back up between them and God, but it does not cost us our salvation if we don't.
Seems that there is a lot of thinking that we have an on again/off again salvation, that is gained or lost every time we sin or repent/confess.
It's the washing of the feet when the whole rest of the body is clean, God cannot bless sin. Seeking forgiveness allows God to continue to work in us and bless us.
So, also in the case of the saved person, there is no forgiveness of sin without confession, but not to the point of costing us salvation. This teaching that saved people are going to go to hell is from the pit. If people were going to hell for their sin, then they're not saved. Hell is what we're being saved FROM.