The yardstick.

Three Standard Stoppages, Marcel Duchamp

"The great heresies in the early Church arose not from the rapid expansion result of these unknown [and untrained] teachers; but in those churches which were longest established, and where the Christians were not so busily engaged in converting the heathen around them. The Church of that day was apparently quite fearless of any dangers that the influx of large numbers of what we should call illiterate converts might lower the standard of church doctrine. She held the tradition handed down by the apostles, and expected the new converts to grow up into it, to maintain it and to propagate it. And so in fact they did. The danger to doctrine lay not in these illiterate converts on the outskirts; but at home, in places like Ephesus and Alexandria, amongst the more highly educated and philosophically minded Christians. It was against them that she had to maintain the doctrine."

... Roland Allen (1869-1947), "The Spontaneous Expansion of the Church and the Causes Which Hinder It", London: World Dominion Press, 1949, reprint, Eugene, Oregon: Wipf & Stock Publishers, 1997, p. 64