To explain I will use a very simple analogy, water. Water is a trinity. Water doesn’t “exist” when it is uncombined oxygen and hydrogen, or does it? Is the term existence even capable of addressing the fact that the essence of what makes water what it is — it’s potential to form in the presence of oxygen and hydrogen — always exists (as long as the universe exists). This essence would correspond with the concept of God the Father. The physical attributes of water that can be measured scientifically would correspond with the concept of a physically incarnate Son of God, Jesus. The non-measurable non-quantitative attributes of water such as how it can soothe, support life, refresh (all subjective attributes dependent on the context an individual finds themselves in) would correspond to the Holy Spirit.
This “Trinity” in Buddhism is called The Three Truths. Westerners most often hear references to this concept when they hear about the “middle way”. However, the dualistic Western approach simplistically defines the middle way as a balance between two factors similar to the dualism that you described in your article as a proto-trinity. The correct meaning, though, is much more profound.
Below is an excerpt from the Nichiren Library to provide more insight:
unification of the three truths ［円融の三諦］ ( en’yū-no-santai): A principle set forth by T’ien-t’ai (538–597) based on the Lotus Sutra. It explains the three truths of non-substantiality, temporary existence, and the Middle Way as an integrated whole, each of the three containing all three within itself. T’ien-t’ai identified this as the view of the three truths revealed in the perfect teaching, or the Lotus Sutra, in contrast to the separation of the three truths, the view espoused in the specific teaching.
Separation of the three truths is the view of the three truths as separate and independent of one another. The truth of non-substantiality means that phenomena have no existence of their own; their true nature is non-substantial. The truth of temporary existence means that, although non-substantial in nature, all phenomena possess a temporary reality that is in constant flux. The truth of the Middle Way means that all phenomena are characterized by both non-substantiality and temporary existence, yet are in essence neither.
The unification of the three truths means that the truths of non-substantiality, temporary existence, and the Middle Way are inherent in all phenomena. T’ien-t’ai taught a form of meditation called the threefold contemplation in a single mind, aimed at grasping the unification of the three truths, eradicating the three categories of illusion, and acquiring the three kinds of wisdom (the wisdom of the two vehicles, the bodhisattva wisdom, and the Buddha wisdom), all at the same time.
Non-substantiality would correspond with the Holy Spirit, temporary existence to Jesus, and the Middle Way to God the Father. (By the way, the wisdom of the two vehicles is the self-improvement derived from learning and self-realization, the bodhisattva wisdom is the self-improvement derived from compassionate action, and Buddha wisdom the eternally existent potential in each living being to be enlightened and thus integrate one’s own happiness with the happiness of all mankind and its environment.)
Average Roman, Greek, and other residents of the Levant were probably more exposed to both Western and Eastern philosophical thought to a greater degree than most contemporary Christians. To them, what became Christian Trinity dogma could have very well been a more satisfactory allegory to the wisdom inherent and readily observable in The Three Truths than the Christian heresies that were repressed.
I hope this little piece of mine has helped you understand how many Westerners who come to practice Buddhism also come to appreciate their Christian roots more than they had earlier in their lives and that indeed claim Buddhist practice is what led them to finally become fulfilled Christians.
Though in our minds, Buddhists like myself, considered Buddhism vastly superior to Christianity in both theory and practice, we, out of an abundance of good manners seldom blatantly assert such superiority in this Christian-centric culture so insecure about the relative strength or weakness of its creeds. I guess I could say the same about the insecurity expressed by the repression of others creeds by Islam, but I have not lived in a Muslim majority culture so I leave that for others to examine. Apparently, the concept of a creator God may be the fundamental failure of both these creeds.