The Westminster 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.
The Westminster Shorter Catechism in Modern English:
Q.16. Did all mankind fall in Adam's first disobedience?
A. Since the covenant was made not only for Adam but also for his natural descendants, all mankind sinned in him and fell with him in his first disobedience.
1. How many persons do we read of in Scripture that represent the human race?
We read of two who represent the human race. The first Adam and the second, Jesus Christ. (1 Cor. 15:45)
2. What reason is given in Scripture that the posterity of Adam fell with Adam?
The reason is found in the covenant of life, in which life was promised upon condition of obedience, and was made with Adam. This was made not only for Adam but for his posterity.
3. Since the covenant was a covenant of life, does this mean that Adam could merit eternal life?
No, it does not mean that Adam could merit eternal life. It was still God's grace that would give eternal life, but a grace that would reward obedience.
4. Was it fair that Adam should represent his posterity?
Yes, it was fair since he was to be the common parent of all mankind, was created perfectly holy, with full power to fulfil the condition of the covenant.
5. How could all mankind be in Adam when he first sinned?
All mankind was in Adam in two ways: 1. Virtually, as a natural root, and, 2. Representatively, as a covenant head.
6. What is meant by saying "all mankind, descending from him by ordinary generation, sinned in him?"
The phrase "ordinary generation" is used to exclude Christ who descended as to his human body from Adam, but not by ordinary generation since he was conceived in the womb of a virgin by the power of the Almighty God overshadowing her.
7. I have always heard, "In Adam's fall we sinned all." Is this a good commentary on this question?
This is an excellent commentary. It should be understood by it that we are sinners first of all because Adam, our representative, sinned for us. Our corrupted nature is the result of our inheritance in Adam.