Why all can only be saved by Faith

And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel. Genesis 3:15

"From the very beginning, long before the days of Abraham, it was

foreseen that God would justify the heathen through faith. 

Faith is the only possible means of salvation; and unless it was the heathen who were justified by it, no one on earth could be justified, since all were heathen. 

*In the fall of Adam, all men became heathen- wanderers from the one true God."
Sabbath School Lesson 1900