You have an incorrect understanding of what justification is.
Yes, on one level it is a static declaration of being Just but not because the charges were dropped but because Jesus paid the penalty for our crime and because He was and is Just, His Righteousness is both imputed as the Protestants hold but is also infused in our soul.
The legal model is only one understanding of Justification. It comes from Paul's letter to Romans. Some say there were Jews living in Rome and the internal content of the letter would seem to support that but, Jew or not, these people were heavily influenced by Roman thought and thus they understood the Mosaic Law in much the same way as they understood Roman Law.
They viewed salvation almost strictly in a juridical paradigm so Paul, in attempt to explain that we are Justified by faith apart from works of the law, talks about legal declaration but even in Romans he makes it clear that when we are Justified we receive the life of grace and Holy Spirit by which we can both overcome sin and, if need be, repent.
In other Epistles, Paul and others use different paradigms to describe Justification.
- In Ephesians Paul talks about being In Christ and also compares salvation to marriage.
- In Galatians, it's more about adoption and divine sonship although we see some of that in Romans.
- In Revelation, the Marriage is played on again
So justification is not just a Not Guilty verdict, in fact, it's not that at all. It's that Christ paid the penalty for guilt.
Now, objectively every sin was forgiven at the Cross when Jesus said it is finished . . . that's past, present, and future, for all men so to use a paradigm or model:
- Jesus took all those sins and left them at the dump.
- When we commit that sin, we go back and pick that sin up and take it home with us and so we must repent in order to receive Mercy that has been there all along.
I hope this helps.