Is this nibbana or god? There is, bhikkhus, a not-born, a not-brought-to-being, a not-made, a not-conditioned. If, bhikkhus, there were no not-born, not-brought-to-being, not-made, not-conditioned, no escape would be discerned from what is born, brought-to-being, made, conditioned. But since there is a not-born, a not-brought-to-being, a not-made, a not-conditioned, therefore an escape is discerned from what is born, brought-to-being, made, conditioned. (Sutta pitaka , khuddaka nikaya , udana 8:3)
That is Nibbana, and not god or Brahman or any kind of Ultimate Reality. It is that which is experienced by the mind, when it is completely free of all fetters and defilements.

I quote from this answer below. Please see that answer for details.


  So, Nibbana is not a thought of the mind, not a concept of the mind,
  not a state of the mind, not a state of consciousness and also not a
  feeling. However, when the mind experiences this Nibbana, which is not
  conditioned, not compounded, not suffering, not impermanent, not
  arising, not ceasing and not changing, it experiences bliss. The mind
  can therefore experience Nibbana, but it cannot feel it or think about
  it.
  
  Sukha or happiness for an unenlightened person is experienced when
  encountering pleasant feelings (from the six senses) or when
  encountering the cessation of painful feelings (from the six senses).
  But for an arahat, sukha or bliss (in this context) is experienced
  when encountering neutral feelings, no feelings and Nibbana.