Book Two - The Knowledge of God the Redeemer in Christ
Chapter 1 - By the Fall of Adam the whole human race was delivered to the curse, and degenerated from its original condition; the doctrine of original sin
Sections 4-7 - Adam's sin entailed loss of man's original endowment and ruin of the whole human race
Section 4 - The history of the Fall shows us what sin is: unfaithfulness
Not just desire for the fruit, but first not believing God's word, assenting to Satan's slander against God, and thus disobeying God's clear test of love. Rom 5:19
Section 5 - The first sin as original sin
Adam cut himself off from union with God, forfeiting life and perverting all things - Rom 8:20, 22. Pelagius tried to say we only imitate Adam, not that we are actually sinful because of Adam. But this was Satan trying to "cover up the disease and thus to render it incurable." Rom 5:12; Ps 51:5; Job 14:4.
Section 6 - Original sin does not rest upon imitation
Adam plunged all his race into his destruction.
Rom 5:12, 17, 19; 1 Cor 15:22; Eph 2:3; John 3:5-6.
Section 7 - The transmission of sin from one generation to another...
...doesn't come "from the substance of the soul or flesh," ultimately.
"Rotten branches come forth from a rotten root."
Though children partially partake of their parents' holiness -1 Cor 7:14,
they were sinful by nature even before that, from conception.