Yesterday I preached from Romans 5:12-21. There is so much in those few verses I wish I had a month to preach on it. The main point, however, is that what we gained is Christ's covenant keeping is far more than we lost in Adam's covenant breaking. It's better because Christ's covenant keeping is better and because the results are better.
Obviously Christ's covenant keeping is better than Adam's. Adam didn't keep covenant, he broke it. But even if Adam had kept covenant (hypotheticals don't make for great theology, I know), Christ's covenant keeping would have outshone his because the conditions were more demanding. John Murray writes, "Christ was called on to obey in radically different conditions, and required to fulfill radically different demands. Christ was a sin bearer and the climactic demand was to die. This was not true of Adam. Christ came to redeem, not so Adam. So Christ rendered the whole-souled totality of obedience in which Adam failed, but under totally different conditions and with incomparably greater demands.” Adam merely had to withstand the test of his fidelity - Don't Eat. He failed. As the second Adam, Christ's tests were far greater, and he met the challenge.
In addition, the results of Christ's covenant keeping are great. Christ's fulfillment of the law's demands and representative obedience doesn't just return us to the amissible innocence of Adam, but confirms us in an unforfeitable righteousness. Adam lived in a period of probation where is faithfulness was tested. Had he passed this period of probation he would have been confirmed in righteousness and earned eternal life. Commenting on this period of probation, Hermann Witsius writes, "man was not yet arrived at the utmost pitch of happiness, but [was] to expect a still greater good, after his course of obedience was over. This was hinted by the prohibition of the most delightful tree, whose fruit was, of any other, greatly to be desired; and this argued some degree of imperfection in that state, in which man was forbid the enjoyment of some good." John Owen writes, "the fountainhead of our race, if he had remained in his first state of sinlessness, would have, at length, obtained a reward for his fidelity, and that reward would have been undisturbed enjoyment of God as was revealed in the terms of the covenant."
Meredith Kline explains how Christ's obedience takes us beyond Adam, the Garden, and the period of probation: "The active obedience of Jesus is his fulfilling the demands of the covenant probation. By the passive obedience of his atoning sacrifice he secures for us the forgiveness of sins. But he does more than clear the slate and reinstate us in Adam's original condition, still facing probation and able to fail. Jesus, the Second Adam, accomplishes the probationary assignment of overcoming the Devil, and by performing this one decisive act of righteousness he earns for us God's promised reward. By this achievement of active obedience he merits for us a position beyond probation, secure forever in God's love and the prospect of God's eternal home" (Covenant Theology Under Attack).
In some ways, the new heaven and the new earth is depicted as a return to Eden, or at least to the Edenlike state of things. At least the presence of the Tree of Life seems to suggest this. However, notably absent is the presence of the Tree of the Knowledge of Good and Evil. It's gone - the period of probation is over. We have passed from an innocence that can be lost to a righteousness that is secure because of Christ's covenant keeping.
Moreover, the image of God initially bestowed upon man is deepened. Adam most certainly bore the image of God, but not as fully as we will in the eternal state. Why would I say that? Because sin was in the realm of possibility for Adam in the Garden, but never for God (nor for us in the eternal state). This really is just a rewording of Augustine. He argued that Adam was created posse peccare, posse non peccare (with the ability to sin, and with the ability not to sin). After the fall, mankind became non posse non peccare (not able not to sin). The regenerate man returns to the Adamic state of posse peccare, posse non peccare (with the ability to sin, and with the ability not to sin). However, we aren't left there but in our transition to glory become unable to sin (non posse peccare).
I'll let Michael Williams have the last word, "Christ's righteousness defeats Adam's sin. God's rightful condemnation over sin is replace by justification. And death gives way to life. Jesus did more than just win back a lost righteousness and a lost relationship to God. Paul ends his discussion of Adam and Christ by proclaiming the surpassing excellence of Christ's work...The result of Christ's obedience...is disproportionately gracious in reference to Adam's disobedience. Through his victory over death, the one true man - who is also the one true Son of God - brings the heavenly life of the resurrection to earth, and we who are in Christ 'shall also bear the image of the man from heaven' (1 Cor. 15:49)" (Far As The Curse Is Found: The Covenant Story Of Redemption).