The Brethren supported themselves by copying manuscripts, and the choice was not only economic. Copying put devotional texts into circulation, occupied the hands while the mind attended to what was being written, and gave houses a product that towns wanted. Several houses moved into printing when printing arrived.
Their characteristic private instrument was the rapiarium, a personal notebook into which a reader copied passages that had struck him, rearranged them, and returned to them. The devotional method that goes with it is slow, repetitive and interior: short passages, taken deliberately, applied to one's own conduct, with a suspicion of speculative theology that runs through the whole movement.
The Imitation of Christ, attributed to Thomas a Kempis, a canon of the Windesheim house of Mount St Agnes, is a rapiarium turned into a book, and it became the most widely circulated Christian devotional work after the Bible. The Brethren also taught, or lodged and supervised, schoolboys in Deventer, Zwolle and elsewhere, and Erasmus was among those schooled in that orbit.
How far the movement prepared the ground for the Reformation is an old argument and not a settled one. John Van Engen's account resists the older reading of it as a proto-Protestant current, presenting it instead as a late medieval experiment in devout life that stayed inside the church it reformed.