Martin Luther (b. November 10, 1483 – d. February 18, 1546) was enlightened to the doctrine of justification when as an Augustinian friar he read Romans 1:17 by the light of the Holy Spirit:
For in [the gospel] the righteousness of God is revealed from faith to faith; as it is written, “The just shall live by faith” (NKJV).
This most precious truth gave him freedom from the condemnation of the Law of God, and also gave freedom to love God for the first time. Under a tormented conscience, Martin had been a frequent visitor to the confessional booth, but never seemed to be able to get true assurance that God had forgiven him (though he regarded private absolution highly). The day he found the gospel there in Romans 1:16-17, he was freed from the fear of condemnation and became the great hero of the Protestant Reformation – restoring light to the Bible for the peoples of Europe. (Yes, that was a simplified version)…
And so we thank God for this (very flawed) man, through whom the chief doctrine of the Christian life was restored to the Church. On this topic, Luther bluntly states
This is the true meaning of Christianity, that we are justified by faith in Christ, not by the works of the Law. This is the highest article of our faith, and if one should abandon it as the Jews do or pervert it like the papists, the church cannot stand nor can God maintain His glory, which consists in this, that He might be merciful and that He desires to pardon sins for His Son’s sake and to save. If this doctrine of justification is lost, the whole Christian doctrine is lost.
This doctrine can never be urged and taught enough. If this doctrine is overthrown or disappears, then all knowledge of the truth is lost at the same time. If this doctrine flourishes, then all good things flourish, religion, true worship, the glory of God, and the right knowledge of all conditions of life and of all things. 
So let us defend to our deaths the doctrine of God’s justification of sinners, based solely on the merits of Christ’s life, penal substitutionary death, and resurrection for our justification. By grace alone, through faith alone, in Christ alone, to the glory of God alone, revealed in Scripture alone. That is our Reformation heritage.
Thanks for reading,
 Martin Luther quoted by Robert D. Preus in Luther and the Doctrine of Justification, Concordia Theological Quarterly, vol. 48, num. 1 (Fort Wayne, IN: Concordia Theological Seminary, 1984), 1.