Why is there just ONE true religion?
This is a sensible question.  One of the well-known causes of pain in world history is the conflict of the peoples of competing religious communities.  To eliminate the existence of exclusive religious communities from the stage of world history would eliminate many wars and their casualties.
This thought, however, raises another interesting pair of questions.  First, how much good would also be lost in erasing the existence of these exclusive religious communities?  Second, would the fraction of world pain being erased come anywhere close to the pain brought by disease and crime?  A third question comes to mind: how much war has been financed and driven in the name of God versus how much was driven by greed for wealth and pleasure?
Is it possible that one of the greatest forces to suppress the criminal tendency in humanity is the moral restraints fostered by exclusive religion?  It seems to be a matter of historical record that the Greek and Roman cultures were highly inclusive and diverse in faith but also very well characterized by legally-sanctioned slavery, murder of unwanted daughters, and ruthless conquest.  In contrast, the gospel of Jesus Christ has slowly and gradually transformed that kind of society into one that values the weak, the merciful, the forgiving, and the dignity of all humanity.  
The history of Christianity has been that it invades each nation and culture not from the top down through power moves but from the ground up through peaceful persuasion. Even though Christianity is an exclusive religion in its message and arrangement, it always enters a cultural frontier as a grass-roots movement, appealing first to the poor and disenfranchised.  This can always be credited to the example of its founder, Jesus Christ, whose story is only told in the Christian Bible.

All Questions