I have been baking bread. Yesterday I made braided bread. I used to make this when I lived alone in a small cabin rental when I first moved north. It has been over 35 years since I made this kind of bread. I remember coming home from hard work in the cabinet shop and stirring up the bread dough. About midnight it would come hot out of the oven. I put butter on it and would eat a whole loaf. Back then it took longer because the store-bought yeast was not fresh. Leaven is a good thing in making soft bread, but leaven in a person is not good. It just takes a little leaven to permeate the whole loaf. Likewise one wolf or false one coming into a fellowship can permeate it with sin and doubts of the scriptures and strife with divisions. If in a Bible study he will attempt to take over and will bring confusion. When a wolf comes my first thought is to separate him from the sheep.
The word for leaven, zumē, is from zeō, to boil, to seethe. It permeates the whole loaf. Usually leaven is used for sin or evil intent.
Mat 16:11 How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees?
Mat 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.
It surprises me how easily a wolf can deceive. Even the disciples were slow to understand when Jesus warned them of the leaven of the Pharisees and Sadducees. I suppose that they were fooled by their outward appearance and their confident speaking. Wolves full of leaven can destroy a nation too, because the people have little discernment. Read the Bible and walk in the light, then you will know the true from the false ones.
Php 4:8 Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.