A little leaven leavens the whole lump! I found that to my cost one year as we were driving to Scotland on holiday. We reached a petrol station about 50 miles from our destination, where annually we always refuel with petrol. After filling the tank I went into the shop, telling my wife to drive the car to a waiting place. But when I came out, the car engine was making an awful noise, and belching smoke. Consternation! After a while I realised that I had put in diesel instead of petrol! 

Providentially, a nearby garage which was on the verge of closing, kindly helped out and was able to drain it, and clean the plugs, etc. This took some hours, but thankfully the car was able to start. It delayed our progress well into the night, all because of a wrong mixture. 

The same is true of doctrine. To mix in error with the truth can cause havoc and impede progress. Indeed, it spawns divisions and sects. One poet wrote, 

Petrol and diesel, diesel and petrol, or in the words of Rabbi Duncan, “wheat and arsenic, arsenic and wheat, it all depends on the proportions.” All error is a disfigurement of truth.