Shorter Catechism Q. 16. Did all mankind fall in Adam’s first transgression? A. The covenant being made with Adam, not only for himself, but for his posterity; all mankind, descending from him by ordinary generation, sinned in him, and fell with him, in his first transgression.
Commentary: Adam, the first human being, and Jesus, who is “the beginning, the firstborn from the dead (Col. 1:18),” are like one another (Rom. 5:14) while also unlike the rest of humanity. They are like one another in that they are both federal (covenantal) heads or representatives of their respective humanities and ages. In this sense, they are both “Adams.” There is “the first man Adam” and there is Jesus who is “the last Adam (1 Cor. 15:45).” The first Adam is the head of the first or old creation and Jesus is the head of the second or new creation. Every human being is connected to one of the two Adams. Either you are “in Adam,” or you are “in Christ (1 Cor. 15:22).” It is in this sense that Adam and Jesus are like one another, and it is what separates them from the rest of humanity. No one else relates to other human beings in this way.
This is why the Catechism rightly says that when God made the covenant with Adam, he made it not only with Adam, but also with his posterity. What this means is that Adam’s actions would affect not just himself but everyone who is connected to him by covenant, which are those who are related to him genetically, that is, “his posterity” or “all mankind.” If the world were a ship, then Adam was its captain. And Adam, as we learned in WSC 13, sank the ship by disobeying God, and so now everyone connected to him by covenant must suffer the consequences of his actions (Rom. 5:12, 15, 17, 18-19; 1 Cor. 15:22). In this sense, we “sinned in him, and fell with him, in his first transgression.”
By nature, every human being is in Adam, and thus is born on a sunken ship, that is born in sin (see WSC 17-19). There is one exception, however, and that is our Lord Jesus Christ. This is why the Catechism says, “all mankind, descending from him by ordinary generation.” Jesus was not descended from Adam by ordinary generation as he was “conceived by the Holy Spirit, born of the virgin Mary.” We are all born in sin, but not Jesus. Jesus did not come into the world in Adam, but as the last Adam, so that we who are in Adam may not perish but have eternal life in him.