See renowned Christian apologist Josh McDowell's answer to the question of why God won't allow unforgiven sinners into His Heaven, in the link at the bottom of this post!
What, exactly, is the big, honking deal about sin that God takes it so seriously that He won't allow ANYONE that hasn't been forgiven of ALL of their sins, through the acceptance of Jesus and His Resurrection, through faith, into Heaven?!!! I mean, we all do bad stuff from time to time, right? Most of us are not murderers, violent or thieves - even though we still do sinful, wrongful and hurtful things (that God deems sin). And we all have friends and family who have done terribly wrong things - and yet, many of us find it in our hearts to forgive others of such things. Especially, as we well know of our own secret sins and the many things that WE ourselves have done that are often terribly wrong. Which is why we don't reasonably expect ANYONE else to be perfect or NEVER do wrong things. Most of us try to forgive others and don't tend to hold them to a higher standard of expectations that we know that WE can't hold up to either.
So, if WE can shrug off the sinful actions of others, and not perpetually hold it against them, why does GOD seem to have a seemingly absurd and unreasonable standard that no UNFORGIVEN sinner can enter Heaven? And to be sure, God most definitely WANTS to forgive us of ALL of our sins (past, present and future) - but he's tied HOW we can receive His forgiveness to whether or not we have received Jesus through faith and commitment to Him - OR whether we, instead, REJECT Him. And lest we forget, the Bible prolifically tells us that Jesus, while ALSO being God in the flesh, DIED a horrific death that He says is the ONLY made our forgiveness possible - but God's forgiveness is only CERTAIN if we will receive and commit ourselves to Him, through faith! But the obvious question is, WHY did Jesus HAVE to die - I mean, couldn't He have just forgiven us, and saved us without suffering such a hideous Crucifixion? God even considers the smallest, microscopic wrong to make us unacceptable to Him. What is this thing about God's obsession with our sin and need for forgiveness? WE cannot cleanse ourselves of (or perfectly avoid) sin! Only GOD can perfect and cleanse us!
See renowned Christian apologist Josh McDowell's answer to why God demands we receive Him AND His death payment made at the Cross, before ANYONE can eternally enter Heaven - EVEN IF, they have only sinned just ONE "tiny" sin: https://www.josh.org/god-loving-cant-tolerant-sin/