Francis Turretin: Justification by faith alone is of the greatest importance

This [doctrine of justification by faith alone] must be handled with the greater care and accuracy as this saving doctrine is of the greatest importance in religion. It is called by Luther “the article of a standing or a falling church.” By other Christians, it is termed the characteristic and basis of Christianity—not without reason—the principal rampart of the Christian religion. This being adulterated or subverted, it is impossible to retain purity of doctrine in other places. Hence Satan in every way has endeavored to corrupt this doctrine in all ages, as has been done especially in the papacy.

-Francis Turretin, "Institutes of Elenctic Theology," 2:633. (Quoted by Morecraft in "Authentic Christianity," vol. 2, p. 954.)