Has Christendom as a whole been affected by false teaching?

Les explains leaven as yeast: when put into dough, it does not leaven just a fourth but leavens all of it. He applies this to false teaching, saying that if you let a little bit of false teaching in as leaven, it is not long until it permeates the whole, and one error leads to another. He then asks what has happened to Christendom in general, and answers that it has been leavened, and Christianity today is leavened. He notes that all the things Scripture and Christianity stood against 50, 60, or 70 years ago are now part and parcel of everyday experience. He illustrates this with a church that kept opening its door to the world, first accommodating some worldliness, then more, until the doors were wide open and you could not tell the difference between the church and the world. His conclusion is that a little leaven leavened the whole lump. He also observes that Paul did not give up on these people but still had confidence in them, trusting they would be brought back to the truth.