If there is one God, why are there so many religions? Philosophers call this the Problem of the Diversity of Revelations. But I was told,
(There is) no reason to think (the) diversity of revelations is a problem, any more than for a therapist to say different things to different clients (whose needs and situations differ).
That analogy didn’t take me very far. The therapist, like a doctor, is giving advice depending on the needs of the client. But God is giving different people contradictory stories about Himself, and also about how they should live. Perhaps God’s messages had to start simple, when cultures were primitive, and became more adequate as cultures developed.
“Lord, do Your revelations progress from lower to higher?”
Yes and no. Much of what I have to say is universal, and good for all times and places. Some is quite specific to the individual and his or her circumstances, the actions he or she faces. Some is developmental, on the side of the culture and also on My side.
“Why not just give everyone the whole truth?”
Your question has presuppositions—that I have given different, incompatible stories to different cultures. This is only apparently true. If you think them through, they are different pieces of the same puzzle. Names shift but that is superficial.
“Even though one says ‘God’ and another (thinking of Buddhism) says ‘Nothingness’?”
No religion puts Nothingness in the place of God. If it appears to, think again. What is the role of each (name)? Is one a substitute or replacement for the other? And (think about) the meaning of each. Are they really incompatible once you examine their properties?
“Perhaps each religion is like a single eye-witness report of some strange event such as a Martian landing. The reports might be wildly different from one another. The challenge would be to sort them out and put them into a single coherent account.”
Not exactly. It’s not to blend the religions into a single synthesis or theology. It’s to put them into one story. (To take your analogy,) imagine a reporter who interviewed everyone who had an encounter with the Martians, starting at the first encounter, and wrote it up as a narrative. Certain consistent themes might emerge, but this would be different from a scientist trying to adjudicate and synthesize the reports. In your version (of the religions), there will be an additional unifying factor—Me.