Justification by Faith

The Protestant doctrine of justification differs from the Roman Catholic, as defined (very circumspectly) by the Council of Trent, chiefly in two points. Justification is conceived as a declaratory and judicial act of God, in distinction from sanctification, which is a gradual growth; and faith is conceived as a fiducial act of the heart and will, in distinction from theoretical belief and blind submission to the church. The Reformers derived their idea from Paul, the Romanists appealed chiefly to James (2:17–26); but Paul suggests the solution of the apparent contradiction by his sentence, that "in Christ Jesus neither circumcision availeth anything nor uncircumcision, but faith working through love."
Faith, in the biblical and evangelical sense, is a vital force which engages all the powers of man and apprehends and appropriates the very life of Christ and all his benefits. It is the child of grace and the mother of good works. It is the pioneer of all great thoughts and deeds. By faith Abraham became the father of nations; by faith Moses became the liberator and legislator of Israel; by faith the Galilean fishermen became fishers of men; and by faith the noble army of martyrs endured tortures and triumphed in death; without faith in the risen Saviour the church could not have been founded. Faith is a saving power. It unites us to Christ. Whosoever believeth in Christ "hath eternal life." "We believe," said Peter at the Council of Jerusalem, "that we shall be saved through the grace of God," like the Gentiles who come to Christ by faith without the works and ceremonies of the law. "Believe in the Lord Jesus, and thou shalt be saved," was Paul’s answer to the question of the jailor: "What must I do to be saved?"
Protestantism does by no means despise or neglect good works or favor antinomian license; it only subordinates them to faith, and measures their value by quality rather than quantity. They are not the condition, but the necessary evidence of justification; they are not the root, but the fruits of the tree. The same faith which justifies, does also sanctify. It is ever "working through love" (Gal. 5:6). Luther is often charged with indifference to good works, but very unjustly. His occasional unguarded utterances must be understood in connection with his whole teaching and character. "Faith" in his own forcible language which expresses his true view, "faith is a living, busy, active, mighty thing and it is impossible that it should not do good without ceasing; it does not ask whether good works are to be done, but before the question is put, it has done them already, and is always engaged in doing them; you may as well separate burning and shining from fire, as works from faith."

Source: Schaff, P., & Schaff, D. S. (1997). History of the Christian church.