1 John 1:9 says, "if we confess our sins.. God cleanses us of all unrighteousness." A Christian's spirit is righteous forever. It has already been "cleansed of all unrighteousness." It has no unrighteousness in it that needs to be cleansed. If it did God would have unrighteousness in Him because we are one Spirit with God. Therefore this can only be speaking to non-Christians who still have unrighteousness in their spirit.
But perhaps you'd say, "Well it is talking about Him cleansing our flesh of all unrighteousness." If that were the case all of our bad habits, lusts, & fleshly tendencies would be completely cleansed from our minds & bodies, instantly renewing our minds 100%. Pretty obvious that that isn't what happens.
Hebrews 10 tells us that a perfect sacrifice, which is what Jesus was, only has to cleanse you once. If confession of sin to be cleansed is repeatedly necessary that would mean Jesus was not a perfect sacrifice & we would need further sacrifices for our sins. If you are a Christian that thinks you must confess your sins to God this one is something to really think about.
Hebrews 10 also says that once you know you are cleansed you are no longer conscious of sins. Therefore the "confessing sins to be cleansed of all unrighteousness" can only be a one time event at salvation because that cleansing, according to Hebrews, is supposed to make you no longer quantify sins or identify things as sins thus leaving you with nothing possible to confess.
Why Christians Don't Need to Confess Sins