The Tempter convinces Eve to eat the fruit using fallacious arguments appealing to her desire for wisdom. Adam joins her in disobedience out of love, leading to the disruption of their once harmonious relationship and introducing mortality into humankind.
The argument used by the Tempter in Paradise Lost Book IX to convince Eve to partake of the forbidden fruit is centered around the idea that eating from the Tree of Knowledge would not bring death as God had warned, but would instead grant wisdom equivalent to that of gods. This argument is not logically sound as it is based on a direct contradiction of God's clear command. However, Eve succumbs easily to the argument due to the allure of wisdom and her own curiosity. As the fruit is described as 'good for food and pleasing to the eye, and also desirable for gaining wisdom', her attraction to it seems inevitable.
When Adam learns of Eve's disobedience, he is initially distraught, but he rationalizes his decision to join her in sin due to his love for her, not wanting to be separated from her even in death. This rationalization is illustrated in the lines where Adam refers to Eve as 'Flesh of my Flesh, Bone of my Bone'.
The first disobedience has enormous impact, evidenced by the changes in their relationship. Initially equal partners, after their sin, Adam and Eve start blaming each other, and their harmony is disrupted. Ironically, their fall from grace, which was sought to make them more like gods, ultimately introduces human frailties and mortality. The poetic lines that highlight the impact include God's proclamation in Genesis 3:22: 'The man has now become like one of us, knowing good and evil.'