Doctrine
Here are a few words about the importance of doctrine in the recent history of the church (by which I mean the last 500 years or so):
It was the authority of true doctrine which shook the whole structure of the Papacy in the sixteenth century and emptied the Roman Church of multitudes of its adherents; it was from the prayerful study of such doctrine at the Colleges of Edinburgh, Glagow and Cambridge that the men who preached in the revivals of the seventeenth century came; it was doctrinal preaching again which resulted in the conversion of thousands in the early days of Methodism; and it was the same heart-acquaintance with theology which characterized all the leaders of the modern missionary movement.
Iain Murray, The Puritan Hope, The Banner of Truth Trust, Edinburgh, 1975, p. 233.
Of course, this is not an idea that was born into the world at the time of the Reformation; in Ephesians 4:11-16, Paul writes that it is through the teaching of apostles, prophets, evangelists, shepherds and teachers—all given by Christ—that the church of God is built to maturity and fullness.