If you were about to purchase a new car ( ie: Ford) and the Ford dealer warned you not to look at other vehicle makes or to read about or believe any information about Ford vehicles unless it came from them, you'd likely be very suspicious. For fear that they were trying to hide something, you might even make of point of going out and doing exactly what they said not to do.
If you were to be that careful about the purchase of a car, why would you not thoroughly research from outside sources a religion that you're about to join and hear what others have to say about it, especially if you believe your eternal life is at stake?