“For as through the one man’s disobedience the many were constituted sinners, even so through the obedience of the One the many will be constituted righteous.”
— Romans 5:19
Adam’s disobedience to God's law in the Garden provides an actual, lived-out record of human disobedience which was counted to be ours through our union with him. His law-breaking is the legal basis on which God justly constitutes all men guilty.
In the same way, Christ’s life of obedience to God's law provides the actual, lived-out record of righteousness which is counted to be ours through our union with Him. His law-keeping is the legal basis on which God justly constitutes guilty sinners righteous.
Christ died for our sins, and He lived for our righteousness. Leave out one or the other, and you preach a half-gospel.