A16: 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 transgression₁.

1. Gen 2:16-17; Romans 5:12,18-19; 1 Cor 15:21-22

For further study see:

1. WLC 22: Did all mankind fall in that first transgression?
   Dr. Dennis Prutow exaplains WSC 16: