How do we know that everyone is sinful and corrupt?

The Scriptures are clear that by nature, all men are sinful and corrupt. As cited in Psalm 14, the declaration 'there is none that doeth good' underscores the total depravity of humanity. This means that in the presence of God's perfect standard, no human can claim righteousness on their own merits. Rooted deep within human nature is a rebellion against God, which results in actions that may appear good by human standards but are in fact abominable in God's sight. Without regeneration by His grace, no one seeks God or does good.