Justin Taylor offers a discussion of the active and passive obedience of Christ. It’s worth a read.
Though some critics of Reformed theology critique the distinction as extrabiblical, I think the New Testament clearly teaches both aspects: the lifelong passive obedience of Christ (his penalty-bearing work of suffering and humiliation) and the lifelong active obedience of Christ (his will-of-God-obeying work) culminate in the cross. Those who trust in him and are united to him do not just have his active obedience credited to their account; nor do they just have his passive obedience credited to their account. The Bible doesn’t divide his obedience up in this way. Rather, believers are recognized as righteous through the imputation of the whole obedience of Christ (the reckoning of Christ’s complete work to our account).
While I am dispensational and not Reformed, I would nevertheless affirm the importance of Christ’s active obedience for the justification of the believer. While Gentile Christians were never under the demands of the Law of Moses, and while the Law of Moses must not be imagined as a mechanism for sanctification, all humans always have been under God’s moral requirements–His Moral Law. Christ perfectly fulfilled these requirements, and is the one human ever to have the right to appear before God in His Own righteousness. That righteousness is credited to us when we believe.