“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.” Matthew 16:12
The disciples misunderstood the figure of speech. Likewise, Nicodemus thought that Jesus was referring to physical birth (John 3:4); the Samaritan woman thought Jesus was talking about physical water (John 4:11); and the Jewish crowd thought he meant eating physical flesh and blood (John 6). When in reality he was imparting spiritual truth. Take care when reading figures of speech in the Bible and ask God to reveal spiritual truth to you.