We are created as amazing creatures with great privileges as God's image. However, we are utterly corrupted in every part of our being that none of us possesses innocence. This true knowledge of ourselves shatters all self-confidence, driving us to God. Only unbelieving, worldly minds will see the greatness in humanity, whereas the believing, heavenly minds will see the ruin, that is humanity. This is all because of sin. The human nature was created innocent by God, but it is now corrupted by sin, and all human beings are naturally evil. It all happened with Adam's fall, and although we don't need to worry about how this original sin is propagated to all human beings after Adam, we can be sure that it is.
Original sin defined: a hereditary depravity and corruption of our nature, diffused into all parts of the soul, which first makes us liable to God's wrath, then also brings forth in us those works which Scripture calls "works of the flesh" [Gal. 5:19].It is important to remember that God is not the author of sin, even though it is difficult for us to know how sin came to be or infected human beings. God did not create human beings sinful, but ever since Adam, every human being, including infants, is sinful and guilty.