Original Sin explained

If Adam passed the test of Gen. 2:15-17, every human would have been born into a paradisiacal state, where the world would not have been cursed because of sin, but instead blessed by God. No one could have reversed that initial state they were born into. In the same, because Adam failed the test, mankind is innately in a state of corruption, along with all creation outside of heaven (i.e. God's dwelling place). In short, Adam's success would have been credited to us, just like his failure was credited to us.

Adam was our federal head, evidenced by the consequence of his actions. Hence, original sin is true, just like original righteousness would have been true.

If Christ Jesus would have failed, all humanity would remain under the permanent state of enmity with God, never to be reconciled, but forever condemned. However, because Christ succeeded, for He was/is impeccable, all of humanity that He represents receives His blessings. Christians are born (again) into a state of peace and righteousness, never to be condemned again. See Romans 5 and Ephesians 2.

Jesus the Christ is the federal head of a new humanity (John 3, 1 Cor. 15, etc), new creatures in Christ, born again and given the righteousness of the perfect Son, God the Son. We get what His perfect life merited, eternal blessings.