Although union language is not the only way of understanding how to attain some form of direct contact with God in this life, it has played a central role in the history of Christian mysticism. The notion that a human being can become one with a god or gods seems to depend on how one views divinity. When the gods are conceived of as stronger beings within the world system, the boundaries between the divine and the human realm are porous, allowing for various kinds of intermingling of superior and inferior beings. The emergence of belief in a single divine source of all things, a transcendent God or Supreme Principle, might seem to preclude the possibility of uniting with that principle. In the face of such transcendence, the proper response of humans should be worship, not desire for communion. Yet paradoxically, the growth of a strong sense of the difference between the one God and the limited world of creatures seems to have encouraged, rather than suppressed, desire to become one with God.

In the wake of the critique of the Olympian gods by Greek philosophers, uniting with the source of all things took on an increasingly important role in Greek philosophical religion. Many of the issues involved in union with the Supreme God were explored by Greek philosophers between ca. 300 BCE and 300 CE. The richest development is found in Plotinus (205–70 CE), whose Enneads set forth a sophisticated doctrine of attaining indistinct union, not only with ultimate existence, or Intellect, but even with the One hidden beyond the realm of all visible and invisible reality. Plotinus’s view of such “oneing” (henôsis) is, at times, expressed in personal terms: “But there is our true love, with whom we also can be united, having a part in him and truly possessing him, not in the flesh from outside. But ‘whoever has seen, knows what I am saying,’ that the soul then has another life and draws near, and has already come near and has a part in him.”