I haven't said that Hinduism is monotheistic.....but the 'millions of gods' is a propaganda for sure....
In reality, we believe in a kind of trinity....it's like, three guys sharing the supreme power-Brahma, Vishnu and Maheswara....
Well not as such the Supreme being is Brahman but if u ask a Vaishnav who is the Supreme being they will say Vishnu if you ask a Shiva chap they will say Shiva but that entity is one which is Brahman.
"That thou art." ("You are the Supreme.") Sanskrit: Tat Tvam Asi. (Chandogya Upanishad 6.8.7)
"The Self (the Soul) is Brahman." Sanskrit: ayam atma brahma. (Brihadaranyaka Upanishad 4.4.5)
"I am Brahman." Sanskrit: aham brahmasmi. (Brihadaranyaka Upanishad 1.4.10)
"Brahman is Consciousness." Sanskrit: prajnanam brahma. (Aitareya Upanishad 3.1.3)
other sayings:
There is One who is the eternal Reality among non-eternal objects, the one [truly] conscious Entity among conscious objects, and who, though non-dual, fulfils the desires of many. Eternal peace belongs to the wise, who perceive Him within themselves—not to others. (Katha Upanishad 2.2.13)
This is the truth which can be found in all major religions
The Vedanta often describes Brahman as Sat-Chit-Ananda. Sat (Existence, Reality, or Being), Chit (Consciousness, or Knowledge), and Ananda (Bliss).
Existence, Consciousness and Bliss are not attributes of Brahman but Brahman Itself.
"Brahman is Sat-Chit-Ananda (ever-present, ever-conscious, ever-blissful)."
SAT (Existence) —
"In the beginning, my dear, this [universe] was Being (Sat) alone, one only without a second." (Chandogya Upanishad 6.2.1 also Aitareya Upanishad 1.1.1)
"All this that we see in the world is Brahman." Sanskrit: sarvam khalv idam brahma. (Chandogya Upanishad 3.14.1)
"Brahman is Reality, Knowledge, and Infinity." (Taittiriya Upanishad 2.1.3)
CHIT (Consciousness) —
"Brahman is Consciousness." Sanskrit: prajnanam brahma. (Aitareya Upanishad 3.1.3)
ANANADA (Bliss) —
"Brahman is bliss (ananda)." (Taittiriya Upanishad 3.6.1)