Luther recovered the gospel, and thereby instituted a truly evangelical revival, when he broke free from the Medieval-Scholastic Nature/Grace metaphysical scheme. In his significant breakthrough insight, faith was no longer understood by him as a natural preparation for grace, as the fulfillment of a condition for receiving supernatural grace by the performance of something that was within man's natural capacity to do. The soteriological scheme of Scholastic theology was synergistic, because Pelagian: God responded to man; man cooperated with God according to his native ability.
Against this Pelagian synergism, Luther insisted on total inability: the utter incapacitation and absolute impotence of the natural man in abject bondage to sin. Faith therefore could not be a condition for grace, for it could not be exercised out of inherently human resources as a natural act performed of sinful man's own initiative for the purpose of man's fitting and preparing himself to be a suitable candidate for receiving grace. Faith itself could only be the result of a prevening supernatural act; it was a free gift of divine grace, resulting entirely from God's unconditioned, monergistic action.
Justification thus was not obtained because of faith, merely through faith—a faith wrought in us. Rather than a cooperation of Nature and Grace (the synergism of mutual effort by both God and man), God acts unilaterally and exclusively, taking the sole initiative in a free act of sovereign grace—grace that is altogether prior to, and productive of, justifying faith. The sola fide arises out of, and is nothing other than, sola gratia.
- Joseph P. Braswell
courtesy: www.reformationtheology.com