Man did not sin because of weakness. Adam was created in God’s image and was holy, fully capable of perfect obedience even in his desires. When Adam ate the fruit he willfully and with full knowledge of the consequences rebelled against God.

We, his descendants, now have inherited the state that he put himself into. Man does not sin because of weakness, as though if by his own power he were able to be holy. No, man sins because he is true to his nature, a nature that is in full, willful rebellion to God. This nature was not produced by God, nor was God responsible for it. Just as cold is the absence of heat, our sin natures were created by one man willfully removing himself from God. God simply allowed man’s free choice to produce its own consequences.