Mother Teresa has come up twice on this thread as an examplar of non-dualism. The view is that she saw God in all beings and because of this Mother Teresa is an example of a Christian non-dualist. I disagree with this conclusion and in the interest of possibly furthering the discussion I'm going to take a few moments to offer a different understanding.
Simply seeing that there is a commonality among all people, or among all things, is not sufficient to qualify someone as a non-dualist. For example, I can say that all humans are made of atoms and molecules, but I don't think from that assertion it follows that I'm a non-dualist. In some forms of Buddhist contemplation we learn to see all people as impermanent and suffering and causally arisen; that is to say we begin to perceive what all people have in common. However, I don't think that it follows from this that those engaged in such contemplations are non-dualists.
Seeing the light of God in all beings is widespread in Christian literature. George Fox, the founder of the Quakers, was famous for this; it is the entire basis for his tradition. There are many other examples. Yet I don't think it follows that Mother Teresa, or George Fox, are non-dualists because of this.
Central to a Christian view are certain pivotal differences between humans and God. First among these is that God is uncreated while human beings are created. This difference is unbridgeable and essential. The light of God in the individual is comprehended by Teresa and Fox as the grace of God guiding the individual towards the uncreated. The light within is not, in this sense, an essence or a true self, because the true self of humans is created, limited, impermanent, while the true self of God is uncreated, unlimited, and eternal.
The tradition of advaita comprehends the limited nature of human beings as illusory (variously defined) while the true self is the same as the eternal and unchanging Brahman. Therefore, "Thou Art That"; meaning human beings are not really limited and mortal. They are really unlimited and immortal, just like Brahman.
Christianity, and monotheism in general, rejects such an equivelency on two grounds. First, it diminishes the grandeur of God (see Saint Anselm) and second, it mistakenly exalts the ego of humans. An Orthodox Priest, a good friend of mine, put it succintly: "This is the start of wisdom; There is a God and I am not he." I'm waiting for some traditional Christian to author a book with the title "I am Not That" (a little joke there).
I think it is a mistake to conclude from the idea that everyone has some aspects of their existence in common, that anyone who holds that view is, therefore, a non-dualist. The crucial, and I believe distinguishing, view of advaita is that this commonality is the only genuine reality. It is possible to argue for the commonalities of people without concluding that these commonalities are the only genuine reality and if one does not draw that conclusion I would say that one is not a non-dualist.