Romans 5:18-19
Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.  For as by one man’s disobedience many were made sinners, so by the obedience of one shall many be made righteous.

Adam, unknowingly, had the future of mankind on his shoulders.  His decision to sin affected everyone who has lived.  But we didn’t choose to be born in sin.  It happens because Adam sinned and sin entered the world and we inherited his sin nature.  It was a free gift, though not one we would want to have.  It is something that comes upon us simply because we are born as human beings into this world.

But the work of Christ undoes that.  By Himself, Jesus had the power to make right what Adam had made so wrong.  Adam made us sinners, but Jesus makes us righteous.  Adam ushered in death, but Jesus gives us life.  Since Jesus is fully righteous, when we are born in Him, we inherit His righteous nature as a free gift, something we do not earn or even deserve.  But it is something given to us by His grace.  His perfection overcomes the sin in us, and we can now be partakers of His righteousness.

The test for Adam was obedience.  God laid out very clearly what he could and couldn’t do in the Garden of Eden, and he chose to disobey.  He chose to partake of the one thing God told him not to partake of.  Jesus, on the other hand, obeyed fully and perfectly everything that God gave Him to do.  He came into the world and lived a sinless life.  He performed all the tasks that were required of Him.  He did that final work of dying on the cross and overcoming sin on our behalf.  And that perfect obedience of Jesus benefits us when we are born by His Spirit into the family of God as a new creature.

Jesus’ work is complete and thorough.  Disobedience separates us from God, but Christ’s obedience draws us back to Him again.  On our own, we have no righteousness, but Christ gives us His so we can be justified and have life, the life that was taken when Adam first sinned.  So just as we didn’t have to do anything ourselves to be born in sin, we don’t have to do anything ourselves to be born into new life.  Thankfully Jesus alone did all that was required, and so we can trust Him fully for this amazing free gift He imparts to us.