11 “But be on your guard against the yeast of the Pharisees and Sadducees.” 12 Then they understood that He was not telling them to guard against the yeast used in bread, but against the teaching of the Pharisees and Sadducees. (NIV)
This warning of Jesus is far-reaching in its application simply because of the power of the illustration of yeast. Jesus had already mentioned yeast in Matthew 13 as He spoke of the spread of the evangelism and the kingdom of God in an organic way.
These two religious groups were castigated by Jesus on numerous occasions for laying the greatest level of importance in life on keeping the law and following the rules. They did these things with cold, calculated precision and relied on their performance to determine their standing before God and the world. Part of the problem with this mindset is the systemic result of losing sight of what is important. So what is important to God? His glory and people. Not a set of regulations or a list of check boxes.
Jesus warns His disciples to be very careful with what they allow “in” them. He is telling them that the way of the religious elite may seem almost harmless, or even beneficial to holiness, if it is “kept in check”, but the reality is much more potent. The legalism that they held firm to works like yeast. It only takes a little in one area of life and it winds up running rampant through every area, changing the structure of everything it touches.
Legalism starts out looking like “good church people” and quickly progresses to unchecked self-absorption, and a dastardly lack of compassion for anyone who doesn’t do things like “you” do them. Paul would also tell the Galatian church the same thing. He actually went 60’s television on them and told them that they had been “bewitched”. Someone, either Samantha or her mother, had cast a spell on them and they had been hypnotized unknowingly by a doctrine of works. This “yeast” robs the cross of its beauty, power, and importance by falsely believing that we can be good enough to exempt ourselves from our need of, as Paul told the Romans, “a righteousness apart from the Law.”
We should never slip into a mentality that is satisfied with rule following for the sake of rule following, or for a public perception (particularly in church culture). We pursue Jesus, the One who never broke the Law; and we walk vicariously in His righteousness as He took our sin and died on the cross for them. One drop of legalism can lead to an entire loaf of pride, selfishness, insensitivity, and coldness. We must pray that God would help us recognize, by the power of His Spirit, this yeast when it tries to infiltrate us.