Since when has every human being been a fallen creature?
Every human being has been a fallen creature since Adam's act of rebellion, beginning with Cain. Adam, as the federal head of the whole human race, plunged everybody into condemnation by eating the forbidden fruit. All are fallen by virtue of Adam's disobedience.
Les explains that as the first man Adam, even with that simple act of disobedience—eating the forbidden fruit—the first man Adam plunged the whole human race into condemnation. Every human being, beginning with Cain, has been a fallen creature by virtue of Adam's act of rebellion. Adam, as the federal head of the whole human race, plunged everybody into that place of condemnation. But God didn't leave it there. He comes around with the second man, the Christ, and makes it possible for every degenerate son of Adam to become a regenerate son of God. Les also connects this to 1 Corinthians 15:45, where the first man Adam was made a living soul, and the last Adam, Jesus the Christ, is a quickening spirit—life-giving. As Ephesians 2:1 says, those dead in trespasses and sin were quickened, made alive.