When we describe human beings, we can separate people into different categories. We can categorize people according to sex: male or female. We can separate them by country of origin: Russian, Chinese, Indian, Kenyan, Brit-ish, American, Colombian, and so on. We can divide them by age: children, teenagers, adults. There are seemingly countless ways that we have devised to group individuals.
Yet with respect to our standing before God, the Apostle Paul knows of only two groupings: people who are in Adam and people who are in Christ. These groups are represented by different federal heads: Adam and Christ, respectively. To put it simply, that means that whatever the federal head does, the people who are in him share the consequences.
Those who are in Adam are reckoned guilty of his sin. Moreover, they also receive a corrupted moral and spiritual nature that inclines them to hide from God and His ways, as Adam did after his fall (Rom. 5:12–21; see Gen. 3). Those who are in Adam also receive a corrupted physical nature; their bodies decay and eventually die. “In Adam all die,” Paul explains (1 Cor. 15:45).
All who are ordinary descendants of Adam are in Adam until by regeneration, faith, and repentance they move from being in Adam to being in Christ (Rom. 5:12–21; 1 Cor. 1:28–30). If we are in Christ by faith alone, we receive forgiveness of sin and our Lord’s perfect righteousness, by which we are accepted by God (Rom. 3:21–26). We also receive a new heart—our moral nature is being renewed such that we more and more die to sin and live to righteousness (Rom. 6:11). But in Christ we will even receive a renewed physical nature. At the resurrection our bodies will be raised imperishable (1 Cor. 15:52).
In sum, those who are in Christ will get back what they lost in Adam, but more accurately, what we will get back will be better than what we lost. As John Calvin comments, “[Adam] by his fall ruined himself and those that were his, because he drew them all, along with himself, into the same ruin: Christ came to restore our nature from ruin, and raise it up to a better condition than ever.” Christ can do this because He is the last Adam, the federal head whom God appointed in His mercy to stand in for us so that we will become the glorified saints God intends us to be (1 Cor. 15:45).