In 2 Timothy 2 Paul describes false teaching as spreading like gangrene.
Think very carefully before you look at the physical effects of gangrene. You will be sickened by it I assure you. And after the wave of nausea has passed you will feel deep pity for whoever suffers from it.
Do you get the point? How bad is heresy? What kind of metaphors are appropriate to convey to us the damage that it does?
It eats away at life and destroys spiritual health. Whatever is vital is destroyed by its corrupting influence.
In that one word Paul directs our thoughts and feelings to the utter horror of false teaching. Never treat it lightly. By using this image Paul is making an impact not only on the mind but also on the stomach. So much for reducing doctrine to abstract ideas. Bad doctrine is not merely intellectually wrong it is also rotten and stinking.
"Paul aptly compares false doctrines with this deadly contagion. For if once they are allowed in they spread till they completely detroy the Church. Since the contagion is so destructive we must attack it early and not wait until it has gathered strength by progress, for then there will be no time to give assistance. The dreadful extinction of the gospel among the Papists came about because, through the ignorance or sloth of the pastors, corruptions prevailed for a long time without hindrance and gradually destroyed the purity of doctrine."
Calvin, Commentary on 2 Timothy