Some people who study mystical experiences have worked to identify the key characteristics of such an experience. What makes an experience "mystical", and what do they have in common? While the definitions are not always fully agreed on by all people, one of the commonly-mentioned features of mystical experience is an "experience of unity".* (Hinman, The Trace of God, p 13). In this experience of unity, "we both become one with the Absolute, and we become aware of our oneness" (Hinman, p13 quoting James).
Is there any real sense in which we are one with the Absolute?
I'd invite the Christian reader to consider a selection from a passage well-known to most Christians as "the parable of the sheep and the goats" (Matthew 25). The Son of Man comes in power with the angels to judge the world. He explains to the blessed: "I was hungry and you fed me, I was thirsty and you gave me a drink ..." (and so forth). He explains to the condemned, "I was hungry and you did not feed me, thirsty and you did not give me a drink ..." (and so forth). Both groups react with complete puzzlement. This is the Divine Judge! When have they ever seen him? When has he ever been in need? Both groups have a similar reaction: "When did we ever see you hungry? ... When did we ever see you thirsty?" And the ultimate surprise for both groups: "Whatever you did for the least of these brothers of mine, you did for me" (or "did not" in the relevant places).
Here is the "experience of unity" turned on its head. It is not a tale of the mystic having a grand moment in which he experiences (or imagines) unity with the Divine. It is the Divine saying that it's true, that there is a unity between the Divine and us that is deeper than we imagine. "If you welcome the stranger, clothe the naked, visit the sick or the prisoner, you have done it for me." And they have "done it for him" more than in the shallow sense that the Divine was merely the motivation while another person received the benefit. No, the Divine claims to be the one who was hungry or thirsty, the stranger, the sick, the prisoner, to whom we show kindness ... or to whom we do not. If the "experience of unity" seeks to be grounded in something that people trust, then the words of Jesus show us how that "experience of unity" looks from the other side of the divide. He also explains how that unity means that we cannot look at our neighbor with indifference.
Another unexpected place where I ran across the "experience of unity" lately was at a blog-neighbor's post on Gaps at the Dinner Table (in which he and some other physicists had dinner, and their conversation turned to God-of-the-Gaps v. Multiverse-of-the-Gaps). One of the dinner companions mentioned "that Monotheism is the ultimate example of a unification hypothesis—explaining diverse things in Nature based on the operation of a single principle." There is a view on which, believing that monotheism is true, we would not be surprised that a transcendent / divine experience often reveals the underlying unity of things. It's a case where, on the premise that monotheism is true, we could reasonably expect an experience of the divine to reveal an underlying unity.
* There is some suggestion to elevate the "experience of unity" to the one essential feature of mysticism, but I'm not convinced that is warranted. The Lukoff study cited (Hinman, p 14) identifies common characteristics of mystical experiences in a fairly comprehensive way without mentioning an experience of unity. I wonder if the experience of unity is similar to the role of nature in mystical experiences: a common feature, but not an essential one.