Our salvation rests not only in Christ’s atoning death, but also in his life of perfect, active obedience. If to secure our redemption Christ only needed to make an atonement for us, he could have come down from heaven and gone directly to the cross. But he also had to fulfill all righteousness by submitting to every point to the law of God. By his sinless life he achieved positive merit, which merit is imputed to all who put their faith in him. Christ not only died for us, he lived for us as well.
Sproul, R.C.; Grace Unknown: The Heart of Reformed Theology; Baker Books; Grand Rapids, MI; copyright 1997; p. 68