"A little leaven" (untruth) "leavens the whole lump" makes the message less believable (robs the gospel of its saving/convincing POWER).
This is why 'hell' does so much damage to the gospel/good news message, effectively hiding it from the world. (It also of course blasphemes God's character every single day.)
If our message based on our belief and perceptions has any untruth in it, it will lack a lot of its effectiveness.
Unfortunately, most human groups, even Christian, have some untruth cleaving to and within them somewhere. And these type things are so very hard to uproot or change.
Every believer, every where (all of us) should always pray when it comes to the gospel and our message, for truth, the whole truth and to have nothing
but truth-otherwise, our witness is undermined and the message does not go as far as it certainly could, because anything false or misunderstood causes the message to not ring true to the hearers.