On reading the Upanishads: they have a certain prestige in the modern world because on their face they are philosophical speculations and not divine revelation. There is no irrationality in them, just profound thought about the most basic issues. The prestige and apparent rationality come at a price, however. As the most sacred scriptures of Hinduism the Upanishads must be viewed as adequate to the world. If philosophical speculations are adequate, then the world is comprehensible by man using his own powers. The world must therefore be such as to allow full human comprehension.
What sort of world is that? The one presented by the Upanishads: an extremely simple world, not even one-dimensional, in which God, man, and everything else are all the same. Each of us can understand God and the world because each of us is God and the world, and salvation consists simply in recognizing the identity.
You make things rational by making them simple, and unless we are given access to a rationality greater than our own the world will be for us either like the world of the Upanishads, much simpler and less real than the world we actually live in, or it will be irrational, which would make our own reason an incomprehensible anomaly. The moral is that revelation is necessary for self-consistent thought that is adequate to our situation.