Then Jesus said to them, "Take heed and beware of the leaven of the Pharisees and the Sadducees."
Matthew 16:6

It only takes a small bit of leaven, or yeast, to take over an entire lump of dough. The effect is gradual, but left alone, it will be total.

The Pharisees were legalistic religionists who sought always to add to God’s Word. They corrupted the Law into something other than that which was handed down to Moses. The Sadducees were liberal intellectuals who sought always to take away from God’s Word. Denying the supernatural, they watered down the truth to make it palatable to natural men.

It doesn’t take much legalism or much liberalism to create a problem. It may begin subtly, adding or subtracting such a small amount of the truth as to be imperceptible. But gradually, both religion and intellectualism will corrupt the entire faith of those who allow it’s intrusion into biblical Christianity.

In Matthew 16:12 it was made clear that Jesus spoke concerning the doctrine of these two groups. Beware brethren. Doctrine matters. We need not be far off, dabbling in outright heresy, to be inviting ultimate error. A little leaven leavens the whole lump.