Many religions based on the same concept of consideration for others and fair play have come to see each other as rivals and do anything but practice what they preach. The Christians during the Crusades set out to eliminate rival religions, and cared not how much murder and mayhem resulted. Islam has become so intolerant of western religions that to reveal one's religious affiliations to be anything but Muslim in some countries is tantamount to a self imposed death sentence. The basis for such polarity is strictly rivalry for territory, followers, and the subsequent control of wealth. To dissuade loyal followers from veering from their path, alternate religions or the countries predominantly influenced by these religions are dressed as evil gluttons, liars, and most certainly not to be trusted. Thus the followers cannot verify the truth of these warnings, as distance must be maintained.
An old ploy, still much in use today.