The mystics tell us that rest and happiness, for the soul and for the creature, "is to be found nowhere but in the one, indivisible unity that is God.... When all that goes to make up a man’s being has become lovingly one with God, then all the soul’s cries are hushed, and the unrest of longing and of acting has ceased" (John Tauler, cited in Magnificat, 8 June 2011). But self-forgetfulness in God is one thing. Does it really mean that the self ceases to "exist" at all – or even that it never existed? That would appear to be the message of Advaita Vedanta and the great sufi Ibn Arabi, and even at times the Dominican Meister Eckhart. But it is a misleading oversimplification, certainly in the case of Eckhart, maybe in the other cases too (see article).
Is Christianity "dualistic"? Philosophers such as Arthur Lovejoy and Robert Bolton would say so, though not in the sense either of extreme Cartesianism (mind-body) or Manichaeism (good vs evil). Christianity is dualistic in that it teaches that God created the world, and that the world is other than God, possessing (received) being of its own. The infinity of the divine Principle means it can create lesser realities without any diminution of or addition to its own substance – and those lesser realities are indeed real: they may be infinitely less than the infinite, but they are infinitely more than nothing. The being of creatures is freely willed in the groundless abyss of the Godhead. That "abyss" or ungrund where the Son is eternally begotten (and us with him in the "ground of the soul") is the source of all freedom, even man's.
This post has been by way of a footnote to the second chapter of All Things Made New. The image, from Wikimedia Commons, shows the Eckhart Portal of the Erfurt church.