Sin is transgression of the law of God. To believe that you (and all other humans) have sinned, you must accept a set of related doctrines, including the existence of the God who made the laws, the wisdom of those laws, and the possibility of an existential punishment for the transgression. Most would also add the ultimate reward for accepting God’s solution to this sin debt, and accepting the solution itself, usually called salvation. As in, salvation from sin. Their Savior, Jesus Christ, is the solution to the sin problem introduced in the Bible, the sacred text of Christianity.
Another way of organizing ideas about evil behavior, unfortunate circumstances, and ultimate ends flows from common sense. Nobody’s perfect, some cruelty perverts morals beyond repair, and societies can increase members’ happiness and wellness by a social contract. Individuals have a choice to make about how much or little to abide by the contract, accepting the consequences when their actions harm themselves, others, or the society beyond what is tolerable to their community. The solutions to such problems could be varied, depending on evidence-based research into causes: mental health care, physical health care, mending social connections, financial assistance, etc. No religion required.
You must give up these common sense ideas about how to organize societies if you accept Christianity as your worldview. Your new guide, your Bible, must supercede common sense whenever the two conflict.
It may seem like common sense that if you are harmed by another person, and they sincerely regret it and apologize, that your choice to forgive and forget ends the problem. But depending on when you lived, the Bible solution involved brutally killing either an animal or Jesus Christ. In the sin economy of Old Testament Israel, your sins would be counted against you in an eventual Judgment unless you confessed them, ritually transferred them into a sacrificial animal, and that animal died in the ritual performed by one of the legitimately chosen members of the priesthood. In the sin economy of the New Testament, it’s the same, except Christ stands in for the priest and the sacrifice.
I used to choose the Christian way of looking at life. Now I reject it, in favor of common sense. Go, and believe in sin no more.