Why do I have to get rid of leaven? I wouldn't know leaven if you threw it in my face. I would prolly say, "hey, why did you throw that paprika in my face?", or ,"why did you throw that crushed oregano in my face?"
Jesus wasn't too crazy about the stuff, but if I don't know what it is and what it looks like, so how'm I supposed to git rid of it? Why?
Leaven is a type of sin.
Paul teaches that various sin can spread and affect others, like yeast can affect the whole lump of bread.
7 Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us.
8 Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.
9 I wrote to you in my epistle not to keep company with sexually immoral people.
10 Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world.
11 But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner--not even to eat with such a person.
12 For what have I to do with judging those also who are outside? Do you not judge those who are inside?
13 But those who are outside God judges. Therefore "put away from yourselves the evil person."
1 Corinthians 5:7-12
Paul teaches us not to keep company with other Christians that are sexually immoral and walk in wickedness.
Their sinful lifestyle can spread and affect the whole Church community.
Likewise false doctrine can spread and affect the Church as well.
12Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees. Matthew 16:12
JLB