Sin is not what we do, it is what we are. Since Adam & Eve's rebellion, everyone has been born with that fatal flaw. There is no one who has ever lived, no matter how good, who has never told a lie, broken a promise, thought wrong thoughts - except one.
Since God's Kingdom cannot accept anyone who is not perfect, and since no one can perfect themselves, God's justice stands against us all. But God is also merciful and loves us. He is not willing for anyone to perish.
Throughout the Old Testament God raised up a special people to be His own, gave them laws and rituals, prophets and priests. Their history serves to prove how impossible it is for mankind to even remain faithful. Sin is the greatest barrier to salvation, not only because it makes us imperfect, but because ourpride will not even admit that we are sinful. People need to understand the bad news of sin and judgement, and accept it, before they are ready to understand the Good News.
Other posts in this series:
2 The Kingdom of God
5 The Old Man
What is a Man?
1 month ago