The Faith and the work of Martin Luther centered in the doctrine of justification. Nowhere did he treat this more exhaustively than in his lectures of 1531 on Galatians. The following two quotes come from those lectures. Both quotes could be considered as answering the question: What is Justification?
“By the one solid rock which we call the doctrine of justification we mean that we are redeemed from sin, death, and the devil and are made partakers of life eternal, not by ourselves … but by help from without , by the only-begotten Son of God, Jesus Christ.”
“The article of justification, which is our only protection, not only against all the powers and plottings of men but also against the gates of hell, is this: by faith alone in Christ, without works, are we declared just and saved.”
Plass, Ewald M WHAT LUTHER SAYS: A Practical In-Home Anthology for the Active Christian 701