That covenant of works was a probation. If Adam kept the law of God for a certain period, he was to have eternal life. If he disobeyed he was to have death. Well, he disobeyed and the penalty of death was inflicted on him and his posterity. Then Christ by His death on the cross paid that penalty for those whom God had chosen.
Well and good. But if that were all that Christ did for us, do you not see that we should be back in just the situation in which Adam was before he sinned? The penalty of his sinning would have been removed from us because it had all been paid by Christ. But for the future the attainment of eternal life would have been dependent upon our perfect obedience to the law of God. We should simply have been back in the probation again.
Here we begin to understand why Jesus’ passive obedience is not enough – if divorced from his active obedience. The passive sufferings of Christ discharged the enormous debt we owe, due to our sins and the sin of Adam. In effect, Jesus’ passive obedience alone would bring our account from hopelessly overdrawn back to a zero balance – our debt would be retired. But having our debt retired and our sins forgiven does not get us into heaven; it simply returns us to the starting point. More must be done if we are to gain heaven. Righteousness must be completely fulfilled, either by us or by a representative acting on our behalf.
Moreover, we should have been back in that probation in a very much less hopeful way than that in which Adam was originally placed in it. Everything was in Adam’s favour when he was placed in the probation. He had been created in knowledge, righteousness and holiness. He had been created positively good. Yet despite all that, he fell. How much more likely would we be to fall – nay, how certain to fall – if all that Christ had done for us were merely to remove from us the guilt of past sin, leaving it then to our own efforts to win the reward which God has pronounced upon perfect obedience.
J. Gresham Machen, quoted from: http://oldlife.org/2013/09/seriously/