Life exists through duality.
The very process of life depends on polar opposites. Life is like a river: for the river to exist, two banks are needed. The river cannot exist without two banks to support it, just as the bird cannot fly without two wings and you cannot walk without two legs.
Life needs duality. Everywhere you will find it, till the ultimate arrives. The ultimate, the absolute, or call it God, transcends duality. But then life disappears. Then you become invisible. Then you have no form, no name; then you have gone beyond. That is Buddha's Nirvana.
These two things have to be understood: life cannot exist without polar opposites. Life is a dialectical process - between man and woman, between darkness and light, between life and death, between good and bad. Life cannot exist without this duality; manifestation is not possible without duality.
Hence, another vision becomes possible to be understood: if duality disappears, life as you know it disappears. Life in time and space disappears.
Then another door opens.
You move to another plane: the plane of the invisible, the plane of the mysterious, the plane where there is no form, no name.
All remains, but unmanifest.
That is called nirvana, moksha: the ultimate state of samadhi where all disappears.
If duality is the way of life, then nonduality is the way of religion - because religion is nothing but an art of transcending the manifested form and going into the unmanifest, going to the original source from where we come and for which we long. Because we belong there.
Wherever we are we cannot feel at home; something remains unfulfilled. We go on missing the original source, because only in the source is rest.