In Mahayana Buddhism, and most prominently in the Vajrayana, it is said that a Buddha has three bodies, the Dharmakaya (the Truth or Wisdom body), the Sambhogakaya (the Enjoyment body), and the Nirmanakaya (the Manifestation body). The Nirmanakaya is the easiest to explain, since it is basically the body that we commonly have in mind when in English we refer to someone's body. The qualification we would add is that this body is to be understood in the context of the other bodies, in other words, this is the body of a Buddha and as such it does not function or subsist within the trap of self-referentiality, but rather it is the living presence, and simply the most readily encountered form, of the truth and wisdom expressed by the other two bodies.
The next easiest to explain, jumping to the other extreme, is the Dharmakaya. This refers to the Absolute, reality beyond all concept, reality itself in its utterly unconstrained, undefinable being. It has no particular form or content (but we would veer into error if we insist that it lacks form). It is incomprehensible and ungraspable. While different enlightened people will clearly have their own personal nirmanakaya, the dharmakaya is all the same. (Actually, we cant quite say whether it is the same or different, but we'll still say that it it is the same - the dharmakaya is the dharmakaya, there is no possibility of distinction or contrast).
Finally we come to the middle term, the Sambhogakaya. Of the three kayas, this is the most difficult to explain because it refers to an aspect of being which is usually conceived of quite differently in our culture. The Sambhogakaya is the realm of symbol and meaning, regarded not as cranially bound subjectivities, but as a real, living, and influential dimension of being. Inner experience, creativity, and form, all pertain to the sambhogakaya. When we speak of sambhogakaya manifestations, we usually are speaking about anthropomorphic appearances of the "celestial buddhas" such as Tara, Chenrezig, Manjushri, etc. But other things such a seed syllables and entire imaginal realms are other manifestations of the Sambhogakaya.
We will now suggest that the Christian concept of the Trinity express an essentially identical teaching. The identity is as follows: The Father = the Dharmakaya, The Son = the Nirmanakaya, and the Holy Spirit = the Sambhogakaya. This is more than just a parallelism; it is the same teaching.
The Father refers to the ungraspable absolute of reality. This is the Dharmakaya.
The Son refers to the incarnate presence of God, a specific person who at the same time is not distinct from the other aspects of the divine. This is exactly what the nirmanakaya is.
Finally, The Holy Spirit is the Sambhogakaya. It is a spirit. It communicates the divine (announcing Christ's glory), revealing its symbolic nature. And it is primarily an inner experience. This is a description of the Sambhogakaya.
To many people, oriented toward thinking in terms only of outer objects and entities, the trinity seems like a hopelessly confused teaching, a mangled attempt at apologetics and a desperate attempt to reconcile a commitment to monotheism with a belief in a number of different divinities. Indeed, much of the convoluted early church wrangling over the theology of the trinity provides fuel for such suspicions. However, this is not the case. Three-in-oneness is an essential truth about our being and existence. In short, we have outer lives, inner lives, and absolute being, with all three of these being aspects of the same life. Christ can be incarnate and still be the Dharmakaya (and thus the incarnate presence of the dharmakaya), because the dharmakaya is the ever-present truth of all being and all people, it is only necessary that he not be closed toward it.
The main difference between the Trikaya system and the Trinity is that Christian tradition asserts that in addition to the fundamental identity of the dharmakaya, that there is only one sambhogakaya and one nirmanakaya manifestation of the divine, whereas in the buddhist tradition there are multiple possible sambhogakaya visions of the ultimate and there are numerous people who have lived in undivided communion with the ultimate. (Actually, Christian tradition does recognize that the saints were also in communion with the divine, but this recognition is uneasy and attenuated, as it attempts to preserve a fundamentally unique status for Christ.)
In this matter the Buddhists are right and the Christians are wrong. This Christian attitude is a form of monotheism gone wrong. As long as God cannot be identified with any particular thing, then saying that God is one is a valid way of expressing the absoluteness of the divine. But to extend this oneness into the realm of manifestation is an error. It probably results from a sly attempt to leverage the idea of monotheism in such a way as to corner the market on religious life and assert a monopoly on the truth, when in fact the truth is all around us if we know how to look.
