Matthew 16:1-12 Heresy From Apostates Is Like Leaven.
The Pharisees and Sadducees, religious leaders of the day, asked Jesus for a sign that he was who he claimed to be. They could read the signs of the weather in the sky, but they could not see the signs that Jesus had already given in abundance, that he was the promised Anointed One, the Christ, the Messiah. It was a wicked blindness, of a group of apostates who had already abandoned the covenant, living contrary to the ways of the LORD. Jonah was a sign to them, because the time was approaching when the gospel would be taken to the Gentiles and they would be cut off from the covenant community, as they were guilty of spiritual adultery (vv. 1-4).
This does not change the fact that the disciples were themselves slow to understand the truth of who Jesus was, and the good news of the kingdom which he came to provide. Even though Jesus had fed huge crowds on two occasions, they nevertheless were more concerned about the lack of bread than what Jesus had just said to the Pharisees and Sadducees. Their first concern should have been the false teaching of these apostates (vv. 5-10). Their worry about bread just revealed the littleness of their faith (v. 8). The leaven of the Pharisees and Sadducees was their false teaching or doctrine (vv. 11-12). They rejected the Anointed One of whom the scriptures spoke.
This continues today. The great majority of the so-called mainline churches also reject Jesus as the Christ, the God-Man promised, and the work he came to perform, and continues to do. Confessions which they call anachronistic, the voice of the one truly holy catholic church, they also reject as being anachronistic. For many, the entire OT and even parts of the New are anachronistic. They seek to modernize Jesus to fit the times, which are increasingly times of pagan renewal, so that their great project is nothing more than a modern version of Christo-paganism. In denying the Son they also deny the Father and the Spirit, the authors of the word that they reject.