I suggest reading C.S. Lewis’ Mere Christianity. It’s a short read that addresses this in very simple terms which I will not paraphrase nearly so well.
Most of us can be nice, upstanding people without religion. Much of what is moral is obvious just by observing human nature. But none of us are perfect. Even the nicest of us have times when we’re petty, selfish, unjustly angry, do small little things on occasion we know we shouldn’t, snap when we don’t mean to, are envious or act jealously, have wrongful feelings. While we may do these things in ways that seem relatively small compared to other massive evils in the world, ultimately they diminish our holiness and goodness and create barriers between us and God. Only in humility and in repenting our sins to God, seeking to be holy, can we truly set foot on the path towards a total transformation to perfect goodness. Most never achieve it in this life. There are many Christians with many failings. Some may have more faults amd vices than some atheists. But in recognizing amd repenting their faults, even if they continually struggle with them more, they are pointed in the right direction towards what they want to be: holy and good. Not just satisfied with where they are, even if it’s “very nice,” and ultimately missing the mark in the end.
