J. Gresham Machen on active obedience

"If Christ had merely paid the penalty of sin for us and had done nothing more we should be at best back in the situation in which Adam found himself where God placed him under the covenant of works. In other words, if Christ only paid the penalty for our sins through his passive sufferings, then we are merely transported back to the Garden of Eden.

“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.

"That is the reason why those who have been saved by the Lord Jesus Christ are in a far more blessed condition than was Adam before he fell. Adam before he fell was righteous in the sight of God, but he was still under the possibility of becoming unrighteous. Those who have been saved by the Lord Jesus Christ not only are righteous in the sight of God but they are beyond the possibility of becoming unrighteous. In their case, the probation is over. It is not over because they have stood it successfully. It is not over because they have themselves earned the reward of assured blessedness which God promised on condition of perfect obedience. But it is over because Christ has stood it for them; it is over because Christ has merited for them the reward by His perfect obedience to God's law.

“Do you see? Christ has passed the test. He has earned the reward. Heaven has been secured by his perfect obedience to God's law. And he did not do all this for himself as if he needed to earn heaven for himself. He did all this for his people - even for you, O believer! On your behalf, he actively obeyed, thereby saving you and placing you beyond the possibility of ever becoming unrighteous again. Your status is secured eternally - what a great hope!"

–Dr. J. Gresham Machen