Why did all of humanity come under condemnation because of Adam's sin?

Adam served as the federal head of the whole human race, so his simple act of disobedience—eating the forbidden fruit—plunged everyone into condemnation. Every human being, beginning with Cain, became a fallen creature by virtue of Adam's rebellion, and thus all humanity shares that condemned standing.

Les explains that Adam, as the first man, acted as the federal head of the whole human race. Even that simple act of disobedience—eating the forbidden fruit—was enough to plunge everybody into condemnation, so every human being from Cain onward is a fallen creature by virtue of Adam's act of rebellion. But God did not leave it there. He brought the second man, the Christ, and made it possible for every degenerate son of Adam to become a regenerate son of God. In 1 Corinthians 15:45, the last Adam, Jesus the Christ, is a quickening spirit, meaning life-giving. Ephesians 2:1 says those dead in trespasses and sin were quickened, made alive.

How did Adam's sin condemn the whole human race?

Adam's single act of disobedience—eating the forbidden fruit—plunged the entire human race into condemnation. As the federal head of humanity, his rebellion made every person, beginning with Cain, a fallen creature. Thus all are condemned by virtue of his act, not their own first sin.