At last they understood that He wasn’t speaking about the yeast in bread, but about the deceptive teaching of the Pharisees and Sadducees. (Matthew 16:12).
Anything that makes us unnaturally big is evil. Whether it be the individual life, or whether it be what is called the work of God, trying to make it bigger than its real spiritual measure, inflating it beyond its genuine spiritual degree, that is something evil, that is leaven. Anything that results in divisions and breaking up - disintegration is evil. Do not let us excuse our divisions. Do not let us look favorably upon the divisions amongst the Lord's people. If there are only two of us who are divided, let us not make excuses for that, let us say, 'This is wrong, this is evil, this ought not to be, there is some evil at work to divide us.' And what may be true between just two people, is true amongst all the Lord's people. We must not look upon divisions as being good, they are bad. And we must know that God is against divisions. He does not accept this working of leaven.
Then what about this leaven that makes things appeal to our natural taste? Paul said to Timothy, "The time will come when they will not endure sound teaching; and they will heap to themselves teachers, having itching ears." And he goes on to say that they will believe the lie instead of the truth. Why is that? Because these teachers appeal to the natural life. These teachers cover over evil. Indeed, they will sometimes call evil good. People do not like the teachers who tell them what is wrong, that this thing and that thing are contrary to God. They like leavened bread, it pleases the flesh... He says, there will be leaven, will be corruption and defilement everywhere, but the teaching of the New Testament is: Keep yourselves pure. See that your garments are not spotted. Walk in this sinful world as those who do not belong to its nature. Although there is leaven everywhere, you be the unleavened bread. But, the world may not like you, the world will not like unleavened bread. It does not please the flesh, but the bread which is pure is pleasing to God.
From: "That They May All Be One, Even As We Are One" - Meeting 30 T. Austin-