Luther insisted that the doctrine of sola fide (Latin for "faith alone"), formed the bedrock of the Gospel. The system that Rome had developed over 1500 years had morphed Christianity from a glorious Gospel of the Incarnate God taking our place on the cross and shedding His blood for the remission of sin to a man-made system that dispenses "grace" in return for human performance. Thus, man once again assumed ultimate responsibility for salvation.
Most Catholics who study their religion try to argue that they look to Christ as their only Savior. Yet they also believe that they receive "grace" as they observe various sacraments, such as confession and penance. These sacraments, as much as Catholics might insist otherwise, incorporate human effort into the acquisition of salvation. Furthermore, the Council of Trent (which convened from December 13, 1545 to December 4, 1563 to repudiate the teachings of Luther, Calvin and other Reformers) boldly condemned the doctrine of justification by faith alone.
If any one saith, that by faith alone the impious is justified; in such wise as to mean, that nothing else is required to co-operate in order to the obtaining the grace of Justification, and that it is not in any way necessary, that he be prepared and disposed by the movement of his own will; let him be anathema. ~~Council of Trent, Chapter XVI, Canon 9Scripture convinced Luther that Christians need nothing more than faith in the finished work of Christ for salvation. Paul's letters very frequently repeat this theme, and his letter to the church in Rome (ironically) first offered Luther a taste of this liberating doctrine. The phrase, "The just shall live by faith" from Romans 1:17 set Luther free from his constant striving to secure salvation. I believe, however, that we need to look at Romans 3:21-26 for a glimpse of how justification by faith comes about.
21 But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it— 22 the righteousness of God through faith in Jesus Christ for all who believe. For there is no distinction: 23 for all have sinned and fall short of the glory of God, 24 and are justified by his grace as a gift, through the redemption that is in Christ Jesus, 25 whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins. 26 It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus. (ESV)As Luther learned, Christ made complete atonement for sin through His blood, requiring only that we trust in Him. Contrary to the Council of Trent's declaration, the real damnation for professing Christians happens when we presume to augment His work on the cross with our own actions.
As we remember Reformation Day tomorrow, we can thank God for using Martin Luther to restore the essential doctrine of justification by faith alone. Without this doctrine, the Gospel loses its power, and Christ wrongly shares His glory (or so we suppose) with helpless sinners. Don't dismiss this foundational doctrine of the Gospel!