The spiritual philosophy of non-duality is increasingly popular today but, while it claims to give a pure and unvarnished description of reality, I believe it rests on a metaphysical mistake. It derives from India, where it is known as advaita, and it basically reduces everything to undifferentiated oneness, regarding anything other than that as, to all intents and purposes, illusion. Thus it is prepared to offer multiplicity a conditional reality for those in a state of ignorance but ultimately sees it as unreal. A subtler grasp of how things are sees oneness and differentiation existing right down to the wire, being two parts of the one whole with the latter not in any way unreal, though still seen in the overall light of God. God is the source of everything, of course, but what he creates is real. It is important to get this right because our understanding of spiritual reality affects everything we think and do and feel and are.
Non-duality is a very easy philosophy to adopt and consequently think one has 'cracked the cosmic code'. It's also convenient in that it does away with God which suits the modern mentality very well. But if understood to mean that there is the One and nothing else has any reality at all, it rests on an error. Certainly all serious religion acknowledges the unity of God and see all life as his but rather than using this to deny the reality in creation a more alert understanding sees that God gives his life to creatures and that they then have their own life, given by him but still theirs, and that this is perfectly real. The absolute oneness posited by non-dualists only exists in the realm of the complete unmanifest and unexpressed. Move one metaphorical (or metaphysical) inch away from that and this oneness, though underlying all, is immediately qualified by other expressions of truth which must always be considered if one is to understand the whole. In fact, for the created being, which we all are, they are in a certain sense primary.
The contemporary Westerner is so indoctrinated with the scientific world view and so much in reaction against traditional Christianity that when he belatedly turns to any kind of spirituality he frequently, wittingly or unwittingly, requires that to fit in with his pre-existing human-centric atheism. Consequently God has become something of a non-necessity in contemporary spirituality which is why Buddhism is so attractive to many people nowadays. It seems to offer spirituality without the disadvantage of God but this is precisely one of its flaws and why, whatever its historical necessity and appropriateness in its original time and place, it is not so suitable for Western people today. The cultural context is different and Buddhism tends to fortify existing deficiencies rather than correcting them as it might have done in the theistic context in which it arose. So much does depend on cultural context which is why Zen Buddhism would have been helpful for culturally conservative medieval Japanese while not being so for beat poets in the 1950s who were reacting against conventionalism already. They needed something with more structure that would counteract their particular excesses and we do too. That is why we need God. Of course everyone, Buddhists included, needs God but he may perhaps be set aside for a while if the concept of him has solidified and the image become more important than the reality which was the case at the time of the Buddha.
Non-dualists and Buddhists don't sufficiently appreciate that our nature is not a unity but a trinity, all parts of which make up the whole. That is to say, they focus on the uncreated part of our being, the part that never leaves the divine world, ignoring that this is only a part of what we are. In reality we are not just spirit overlaid by various unreal 'sheathes' but spirit, soul and body, all of which contribute towards the wholeness of our being, and if we deny any part of this then we are not living in truth. To mistake the highest part of our being for the only real part is an error which you might categorize as a kind of intellectual absolutism. In fact, our spiritual goal is not to return to unmanifest existence as though our life in a body was an irrelevance but to learn the lessons of incarnation in a material world, the primary lesson being that of self-sacrifice in love, and this we can only do by giving all parts of reality their full significance.