Beware of the leaven (yeast) of the Pharisees and the Sadducees

Matt 16:6 Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees.
Matt 16:12 Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.

Here Jesus warns His disciples to beware of the leaven of the Pharisees and of the Sadducees. Verse 12 shows that the leaven speaks of the doctrine of the Pharisees and the Sadducees.
The Pharisees and Sadducees were the religious people of Jesus’s time. We see that Jesus was not against the sinners, prostitutes and tax collectors, but He was against these religious people. Doesn’t that seem strange that He would do so? If Jesus was here physically on this earth today, what would He say about the modern church?
The doctrine of these religious people was that they could attain righteousness by doing the works of the law. In John 5, Jesus tells them that they were seeking eternal life in the scriptures, but they failed to understand that the scriptures were pointing to Him as the only way to salvation or eternal life. They were looking for “do’s and don’ts” in the scriptures to be able to fulfill them and thereby attain righteousness by their performance. Jesus called them “hypocrites, snakes etc.” They observed the laws outwardly but their inside was full of darkness. Jesus shed light on the righteous standards of the law and showed every single time that the religious people fell short of those standards. So rather than falling on God’s mercy and grace they wanted to trust in their own performance to attain righteousness. Doesn’t this sound like the modern church? They try to grow in righteousness and holiness by their own works rather than depending on the grace of God who is the indwelling Christ in every believer. They try to comb the scriptures to find every single law from Genesis to Revelation and make additional laws and put this heavy burden on the people.
The Apostle Paul in his epistles lashed out against such religious people who wanted to mix their legalism with the pure gospel of the grace of our Lord Jesus Christ. He calls them “wolves.”Writing to believers in the Galatian church, he asks them, “You foolish Galatians, who has bewitched you? After starting your Christian lives in the Spirit, why are you now trying to become perfect by your own human effort?”In chapter 5 He tells them that, “if you are justified by the law(performance), then you are cut off or severed from Christ! You are fallen from grace!” That’s a very serious statement! He further tells them that “a little leaven leaveneth the whole lump”. A little leaven or doctrine of legalism (even a little bit) will leaven the whole lump. Grace and legalism cannot be mixed.
We are saved by grace, made righteous by grace, live by grace, kept by grace, sanctified by grace, perfected by grace. This is the pure unadulterated message of the gospel of grace. Rom 11:6 says, that grace is no longer grace if your works are involved.
In conclusion, let us not nullify (invalidate) the grace of God (by mixing it with legalism), for if righteousness comes through the Law, then Christ died needlessly. (Gal 2:21)