The tone is the whole achievement here. This is not doubt and it is not blasphemy. It is the exasperation of somebody who has been let down repeatedly by a person he has no intention of leaving, and anyone who has had that relationship with a human being will recognise the register immediately.
The first half is built out of arguments that fold back on themselves. If nothing exists except you, why bother with a veil, and who is being hidden from? If negation itself points at you, then denying you is a kind of proof. If I never imagined you, where did I get the idea? Each one is a small trap, and each has the same shape: the poem catches God in a contradiction and then, rather than concluding that he is not there, concludes that he is a puzzle. Everything depends on that distinction.
The frustration running through the whole poem has a name by the eighth section. Come into my hands and you are an idol; stay out of them and you are God. Anything that can be held is a made thing, and the one quality keeping God from being an idol is being out of reach. So the complaint and the devotion are the same gesture: what he cannot reach is the only thing he would bother reaching for.
The second half stops arguing and starts listing, and the list is a strange one. Jesus given the power to raise the dead and then hung on a cross. Moses asking to see, and the mountain set alight. Abraham thrown into Nimrod's fire, which is then turned into a rose garden. Joseph dropped down a well, sold in a market, made ruler of Egypt. Mansur taught to say I am the Truth and then executed for saying it. The same hand does both things every time, and the complaint is not that God is cruel, only that there is no telling which way it will go. Then the list turns homeward, to Punjab's own lovers. Ranjha scattered in the scrubland at Jhang, Qais turned into Majnun, Sassi burnt up in the Thal, Sohni carried off by the river. All four went looking, all four were destroyed by the looking, and they are filed in exactly the same column as the prophets. Anyone who came looking is on the list.
And then the ending, which is why the poem never turns bitter. When you feel like it, he says, you call your beloved right up to the throne and grant him the whole ascension in a single night. The same power that burns the mountain does that too. There is no rule, no method, nothing anybody can do to make it come out one way rather than the other, and that, finally, is what he means by an impossible puzzle. Nusrat Fateh Ali Khan's recording is the one everybody knows, and live it runs well past an hour.