Jesus teaches his disciples to guard against the leaven of the Pharisees and Sadducees (Matt. 16:6), by which He meant their teaching or doctrine. This refers to false teaching, but a similar point is made concerning the leaven of the Word of God (Matt. 13:33). Doctrine is leaven, and it works like leaven. This means that it works slowly.
Two points of application. Ministers of the gospel should expect their teaching to have transformative power over the course of years and decades. Gospel proclamation is not like the coach’s half time talk in the locker room, where the exhortation is all about now, and the application, all the application, immediately follows. There are decisive points where obedience is called for, of course, but do not underestimate the power of godly teaching to transform all kinds of things in ways that we would consider oblique or indirect. You might see a man do the right thing today, and not know that it could all be traced back to a sermon heard by his great grandmother on his mother’s side in 1932.
The flip side of this is that to be on guard against the leaven of the false teachers means that the guardians must warn against dangers that are not immediately apparent. The problem with some of the conservative watchdogs in the Reformed world is not that they do this. It is that they do it so badly.