One without a second is a simile used in context of Brahman, the one and only God Almighty, creator of entire cosmos. As detailed in Vedas (Hinduism revelations), Upanishads (independent spiritual treatises) and also Bhagavad Gita Brahman truly is one and only one!
The present cosmos (called Brahmaand in Hinduism) came into being when the only Brahman exploded itself with big bang. Until explosion of big bang there was no entity like present cosmos. What existed before the big bang were remains of old cosmos after its complete dissolution.
When dissolution of a cosmos occurs, the entire cosmos slips into a huge black hole reducing to size of half a thumb. In absence of dross impurities within all souls’ atmans in cosmos occupied the volume of half a thumb. This magnanimous primordial cosmic energy, cluster of all pure souls’ atmans in cosmos is what we called Brahman in all its glory.
Before present cosmos came into being existed Brahman and even after occurrence of big bang exists Brahman. This proves the saying that Brahman exists from times immemorial. As souls atmans are indestructible, could not be created or destroyed, they also exist from times immemorial, beginning of time.
Brahman (God Almighty) also called Parmatman is one without parallel. God is primarily referred to as God Almighty because it causes explosion of big bang, origin of cosmos. Nothing was impossible for this cosmic entity, something beyond comprehension of most human beings.