Justification, the Heart of Faith

There is this about the article of grace that if one diligently and sincerely remains loyal to it, it keeps one from falling into heresy and from undertaking anything against Christ or His Christendom. For with it comes the Holy Spirit, who enlightens the heart by it and keeps it in the true, certain understanding, so that it is able precisely and plainly to distinguish and judge all the other articles of faith and forcefully to sustain and defend them. One plainly sees this in the ancient fathers. When they remained loyal to this article, based their doctrines on it, and drew them from it, they remained altogether pure in all articles. However, when they departed from it and argued without it, they, too, went wrong and stumbled badly.... For when the knowledge of Christ is gone, the sun has lost his brightness, and darkness supreme reigns.  Then one can no longer correctly understand, and one cannot protect oneself against any error or false doctrine of the devil.

Source: Quoted in Ewald M. Plass, What Luther Says, A Practical In-home Theology for the Active Christian.  St. Louis: Concordia Publishing House.  1959 Edition, 10th Printing (1994), p. 703.