But I really must decline to speculate any further about what might have been if Christ had done something less for us than that which He has actually done. As a matter of fact, He has not merely paid the penalty of Adam’s first sin, and the penalty of the sins which we individually have committed, but also He has positively merited for us eternal life. He was, in other words, our representative both in penalty paying and in probation keeping. He paid the penalty of sin for us, and He stood the probation for us.
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.
J. Gresham Machen, The Doctrine of the Atonement: Three Lectures