The first instance of monism in organized religion, Brahman is the root source and Divine Ground of everything that exists (and does not exist!) in Hinduism. It is defined as unknowable and Satchidananda (Truth-Consciousness-Bliss). Since it is eternal and infinite, it comprises the only truth. The goal of Hinduism, through the various yogas, is to realize that the soul (Atman) is actually nothing but Brahman. The Hindu pantheon of gods is said in the Vedas and Upanishads to be only higher manifestations of Brahman. For this reason, "ekam sat" (all is one) and all is Brahman. This explains the Hindu view that " All paths lead to the one Brahman, though many sages [and religions] call him different things."
"Om. That supreme Brahman is infinite, and this conditioned Brahman is infinite. The infinite proceeds from infinite. Then through knowledge, realizing the infinitude of the infinite, it remains as infinite alone." --- Mandukya Upanishad