Galatians 2:15-21

But when the Apostle says that man is justified by faith and freely, these words are to be understood in that sense in which the uninterrupted unanimity of the Catholic Church has held and expressed them, namely, that we are therefore said to be justified by faith, because faith is the beginning of salvation, the foundation and root of all justification, without which it is impossible to please God.  - Council of Trent, Roman Catholic Church
After finishing his arguments against his assailants, Paul turned the message of his letter to the gospel and the nature of saving faith. When the legalist Judaizers had come into the churches of Galatia, they had not only sought to undermine Paul’s character and authority, they had labored extensively to undermine the gospel that he preached. Through deception and slander they eventually were able to turn entire congregations of believers away from the message that Paul had taught.
At first they affirmed some of Paul’s teaching, namely, the need for a man to be justified before God. Justification is the act whereby God declares wicked sinners who are his enemies to be forgiven of their sins and accepted in peace as His own sons and daughters. The legalist Judaizers agreed with Paul that Jesus died on the cross to bring us justification with God. By taking on himself the sins of those who believe in him, bearing God’s wrath as a substitution for their sins, and then by offering them his own sinless life of righteousness, Jesus accomplished justification for all those who believe. This was the message of Paul, and this point was not a point of contention with the Judaizers.
After affirming Paul, however, they attacked Paul’s premise of “through faith alone.” They, like the Roman Catholic Church, argued that the work of Jesus was “the beginning of salvation, the foundation and root of justification,” but that the work of Jesus was not all that was needed for justification before God. They argued that after one placed his faith in Jesus, he could eventually be justified by obedience to the Law of Moses. Jesus wasn’t enough. His sacrifice was good enough to get you started on obeying God, but ultimate justification depended on you.
This view is not only horribly wrong, but it is dangerously devastating. Any who would turn away from Christ alone as their only hope, and see Christ instead as a mere means whereby their own works become the method to being justified, these people are not truly justified, and the wrath of God will come on them. Only through faith in the work of Christ can we have peace with God. Faith is not a foundation out of which grows a root of justification; it is the fountain through which flows full justification in Christ alone, by God’s grace alone.
Paul argued to the Galatians that any who would say they were justified by the work of Christ, and then press on to further justify themselves by their own works were showing that Jesus did not fully cleanse them from their sin; rather, he only took them further into sin that they still needed to be saved from. Paul also argued that those who think that works are required for salvation are saying that the sacrifice of Jesus is inadequate for salvation. If His sacrifice was inadequate, then Paul argues in verse 21, it was a waste for Jesus to die. The death of Jesus is not a waste; it is the only hope for our justification.

Food For Thought: What is wrong with the statement by the Council of Trent listed above?