He warned them about religious leadership. He called what they did leaven. Leaven in the Bible is mentioned 22 times in the Old Testament and 17 times in the new. Leaven/yeast is basically old, fermented dough. It nearly always means the incursion of Sin, Evil, Compromise and giving in to base impulse as Herod did Mark 8:15. Jesus also spoke of false teaching as leaven. Matthew 16:6-12 He also spoke of Leaven as Hypocrisy. Luke 12:1
If you associate yourself with such people, you will find them mostly in churches, they will destroy your spiritual walk. You will end up shipwreck. 1 Corinthians 5:8
I know this sounds harsh, but there are many so called Christians who have the capacity to destroy you spiritually. Suck the life blood out of you, like a vampire.
I know this effect. I have experienced it. If you are part of a spiritless house, one that is simply religious, you will die spiritually. Sure you can watch some TV, read some books, listen to CDs, but in the end, you will die. We need one another. We need the power of love and a spiritual transfusion of life blood.
Beware of the spiritual Vampires. They look like Christians on the outside, but inside are dead tombs. It was so with the Pharisees, it is so with Many in the church today. Look out for them, they are vampires:
"What sorrow awaits you teachers of religious law and you Pharisees. Hypocrites! For you are like whitewashed tombs--beautiful on the outside but filled on the inside with dead people's bones and all sorts of impurity. Matthew 23:27