Why would there be sins if there is no Judge, no One who declared what is moral and what isn't?

There is a very old saying that is also good to ponder, imo.

If there is no God (or Judge) then when we die, we are all fine (those who also believed there was a god.)

If there IS a God, and He requires accepting this, then those who don't believe there is a God have a real problem when they die.

The good part about this, imo, is that once you do accept that there is a Supreme Being, you know (inside)that you made the right decision.