For seven days you are to eat bread made without yeast. On the first day remove the yeast from your houses, for whoever eats anything with yeast in it from the first day through the seventh must be cut off from Israel. Exodus 12:15

It must not be baked with yeast; I have given it as their share of the food offerings presented to me. Like the sin offering and the guilt offering, it is most holy. Leviticus 6:17

“Be careful,” Jesus warned them. “Watch out for the yeast of the Pharisees and that of Herod.” Mark 8:15

Your boasting is not good. Don’t you know that a little yeast leavens the whole batch of dough? 1 Corinthians 5:6

Yeast is a fungus, a foreign substance that reacts with whatever it is stirred into to cause a change.

When God prepared the Israelites to leave Egypt, he commanded that they did not bake bread with yeast in it. Why? Yeast takes time to infiltrate the bread and causes it to rise. If the people baked bread with yeast in it, while God prepared an escape plan for them, they would saying "that's nice, God, just as soon as this bread is done, we'll be on our way."

In other words, the things of this world were more important than the things of God. When God called them out of Egypt, they would rather wait for bread to rise than to get out of slavery?

Yeast takes time to work.  However, we must be obedient to God the moment he calls.

Yeast also represents sin symbolically. The sin offerings of grain and bread could not contain yeast since it represented a foreign substance that caused a change in whatever it was put into contact with. Sin does that too. It “puffs us up” thinking we are larger than life, causing our egos to rise.

Jesus used yeast to talk about the influence of the Pharisees and Herod to contaminate the religious faith of others. Paul in 1 Corinthians warns that a little selfish boasting can infect the whole person.

God does not hate yeast. But he does hate sin.