What did the leaven represent? Today, "the whole mass" represents all the nations, and the fermenting represents the spread of the Kingdom message by means of the preaching work. In 1975...
Today, what permeates the whole mass of the churches of Christendom? True Bible teaching and structure and procedure and observance? No! It is the fusion religion that Pontifex Maximus Constantine promoted, in which the Babylonish doctrines and procedures are the fundamental things rather than the teachings of God’s inspired Holy Word. Constantine was the one who as presiding officer of the Council of Nicaea settled the dispute over the personality and attributes of Jehovah God by ruling in favor of the Babylonish doctrine of the Trinity. Jesus Christ foretold this process of corrupting Christian doctrine and practice by giving the parable of the leaven. He said: “The kingdom of the heavens is like leaven, which a woman took and hid in three large measures of flour, until the whole mass was fermented.”—Matt. 13:33.
9 The fermentation of the whole mass of Christendom has now had sixteen centuries of time in which to take place. Who can deny that Christendom is completely leavened today by the corrupting influence of Babylonish doctrine and worldliness and Nimrod-like defiance of the universal sovereignty of Jehovah God? This corrupting of the tremendous mass of Christendom’s imitation “sons of the kingdom” has made the false earthly “kingdom of God” an excellent place for the agents of Satan the Devil to take refuge in, like the “birds of heaven” lodging among the branches of the full-grown mustard-seed tree.—Matt. 13:31, 32.
--w75 10/1 p. 597 pars 8, 9