As in the chain of salvation justification follows calling (Romans 8:30) and is everywhere set forth as the primary effect of faith, the topic concerning calling and faith begets the topic concerning justification. This must be handled with the greater care and accuracy as this saving doctrine is of greatest importance in religion. It is called by Luther”The article of a standing and a falling church” (aticulus stantis et cadentis eccleisae)1. 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. For this reason, it is deservedly placed among the primary causes of our secession from the Roman church and of the Reformation.
—Francis Turretin, Institutes of Elenctic Theology, 16.1.1
1. It was not Luther who said this but J. H. Alsted. It’s interesting to see, however, that it was received knowledge in the mid-late 17th century that this saying belonged to Luther.