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15. What was the sin whereby our first parents fell from the estate wherein they were created?

The sin whereby our first parents fell from the estate wherein they were created was their eating the forbidden fruit.

16. Did all mankind fall in Adam's first transgression?

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.