How do you describe an elephant?

One of the arguments against the exclusivity of Christianity is that each religion sees part of the spiritual truth but none can see the whole truth. The use the example of the blind men that try to describe an elephant by the part that they touch. One says it is long and flexible like a snake as he holds the trunk. Another says it is like a tree trunk as he feels the leg. A third says it is flat like a wall as he examines the side.

So what is the problem with this example? The problem is that it is told from the point of view of someone who is not blind. How could you know that each blind man only sees part of the elephant unless you claim to see the whole elephant?
So "how could you possibly know that no religion can see the whole truth unless you yourself have the superior, comprehensive knowledge of spiritual reality who just claimed that none of the religions have." Tim Keller in Reason for God.