Read Mark 8.
Do you remember the Judiazers from passages we’ve already read in Acts, Galatians, and Colossians? They were a group of people who called themselves Christians but tried to impose Old Testament ceremonies on the Gentile believers who came to Christ and became part of the church in the cities where Paul traveled.
Here in Mark 8:15, Jesus forewarned the disciples about the Judaizers when he said, “‘Watch out for the yeast of the Pharisees….”
It seems surprising that Jesus would need to warn the disciples about the Pharisees. They were a constant problem for Christ during his ministry on this earth, so I would expect that the Twelve would be wary of them. Maybe they were; however, we need to remember that the disciples grew up in synagogues that were dominated by Pharisaic leadership and interpretation of the Law. While the disciples may have distrusted the Pharisees based on their experiences with Jesus, they were probably sympathetic to the outlook on life and spirituality that the Pharisees had.
Jesus warned the disciples that the teaching of the Pharisees (and Herod, but that’s a different story) would be like yeast. I don’t know anything about baking but I am told (like, here) that a small lump of yeast will grow and spread throughout an entire batch of dough. A little Pharisaism, then, in the church would grow and permeate the whole congregation. So Christ warned the disciples not to let them and their rules into the church.
There are some groups of Christians who would like to bring the church back under observance of the law. Our church, however, is more likely to be infected with Pharisaic attitudes than classic Pharisaic theology. We might never tell a newly converted man that he needs to get circumcised and stop eating ham. But we might be tempted to try to impress others with our pious words in prayer or with our extravagant giving to the church. We might never try to revert to observing the Sabbath, but we might judge someone for not wearing the “right” clothes on Sunday morning.
Do Christians need manmade rules to keep us from sinning? Maybe we do and we shouldn’t judge another believer who has different convictions about this or that than we do.
But we also shouldn’t judge other Christians if they are living obediently to God’s word but apply it specifically in different ways than we do. That is a Pharisaic attitude and once it infects our hearts and our church, it will grow and spread until it permeates the whole congregation.
This is true of all false doctrine, actually. How much error is good for your Christian life? How much false teaching can a church tolerate and still be healthy? According to Christ, not much. That’s because false doctrine spreads. So, we need to know our theology well and never dabble in or tolerate bad theology in our lives or our church family.