Guard againt yeast of Pharisees and Sudducees

Matthew 16:1-12
The Pharisees and Sadducees came to Jesus and tested him by asking him to show them a sign from heaven. Jesus said none will be given it except the sign of Jonah. You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times.
Guard against the teaching of the Pharisees and Sadducees. Working of the yeast is underground, could not be seen.
Jesus will rise from the death three days later like Jonah. There is none sign could be seen.
The Pharisees had demanded a sign once before. Now the Sadducees, the ultra-conservatives, join them in putting Jesus to the test. But his answer is still the same.
Why?
The yeast is like to see the evidence from the miracles, but it is the faith to Jesus. If teaching like Pharisees and Sadducees, they are following the law can not find the eternal life, but lost the heart of kingdom of heaven. Try to find the truth will be lost.
Yeast/leaven : An evil force, working unseen. The disciples must not be like those Pharisees and Sadduccees whose hostility(敵意) prevents them from recognizing the work of God.
How?
Don't let any evil mind came into our heart, evil fruits could be growing up.