Note: There is much more to say about The One, and this section will be expanded in the future.
In the Pythagorean tradition the many individual Gods, such as Zeus and Hera, Apollo and Artemis, are understood as offspring of the Father and Mother of the Gods, commonly known as Kronos and Rhea (see Part I, Theogony). These two rule all the principal dualities of the universe, including light and dark, form and substance, mind and matter, being and becoming, identity and diversity. These dualities are all manifestations of Unity (the Monad) and Plurality (the Indefinite Dyad).
However, beyond the Father and Mother of the Gods, most Pythagoreans recognize a primordial Divinity who encompasses all Divinities, and indeed all things in the universe. This transcendent Deity is sometimes called Aiôn ("Eternity" or "Unlimited Time"), but also The All (To Pan), The Whole (To Holon), and The One (To Hen). Indeed, Proclus (c.411-485 CE) says, "The One is God" (To Hen Theos). (I will use all these names, as appropriate.)
According to the myths, Aiôn is bisexual and by self-fertilization gives birth to the Father and Mother of the Gods. But since Kronos and Rhea together create ordinary (determinate) time and space, Aiôn's creation of Them is outside of time, an eternal Emanation. Such a Deity might seem remote indeed from our everyday lives, but I will try to explain why, for Pythagoreans, union with The One is the essence of a spiritual life.
Heraclitus (fl. c. 500 BCE) says, "God is day night, summer winter, war peace, glut hunger (all the opposites, that is the meaning)" (fr. 67). That is, the Whole transcends duality. The All, by definition, includes everything; therefore it must include all opposites. The One, by definition, must unify everything; therefore it must unify all opposites. Hence, The One is paradoxical and contradictory, and the ancients called It Ineffable, Invisible, Unspeakable, Unnamable, and Unknown.
The Whole is simultaneously mind and matter, one and many, stability and change. Indeed, It is and is not, for It unifies the opposites Being and Not-being (see "Enigmas of the One" in Pt. III). It encompasses not only what actually is, but also all potentialities, all possibilities. This is the foundation of Divine Providence (Pronoia), for The All comprehends all that might be, and out of that totality manifests what is. Further, because The One is the source and origin of the order and beauty of the Cosmos, and because It is the goal towards which everything is striving (the end of all becoming), many Pythagoreans identify The One with The Good (T'Agathon).
However, just because The One is called "The Good" and "The Highest" (To Anôtatô), it should not be supposed that It stands above something else (e.g., matter or the physical world), which is evil and debased. First, this is impossible, for The One is The All, and so there is no "something else" other than The Good. Furthermore, Pythagoreans recognize that transcendent Ideas emanate throughout the things of which they are the Essence; for example, Life pervades all living things. Therefore the highest Idea, The One, emanates Its Unity and Goodness throughout all things (existing or not existing). That which all things have in common, the featureless substrate of everything (existing or not existing), is Primary Matter (Prima Materia, Hylê), which is Universal Potential, the ultimate Principle of Receptivity (as the living One is the principle of activity). But be careful: Matter is not something different from The One, for The One is All; Matter is just The One from a different perspective. Matter is good because The One is good, for They are the same. Each of these two apparent opposites is called "The Extreme" (To Akron) and "The Simple" (To Haploun). (See "The Mother of the Gods" and "The Goddess of the Cosmic Tree" in Part II for more on Matter.)
In particular, we see that mind and matter are not two mutually exclusive categories; rather, The All is Mind and Matter simultaneously. The physical world is familiar (more or less), but the other side of The All is the psychical world, the Mind of The All, of which our individual minds are parts.
We are fooled into thinking that our individual minds are separate and independent, but that is because we identify ourselves with our ego-consciousnesses. However, the ego-consciousness is just the most concrete, material layer of the mind, evolved to facilitate our functioning in the physical world of space and time. Deeper or higher (pick your metaphor) than the conscious mind lies the unconscious, the Higher Self. (Although it is beyond consciousness, we can learn about it indirectly.) At the furthest reaches of our unconscious minds we reach the collective unconscious, which we all share. That is, our individual Higher Selves are just organs of the one Highest Self, the Mind of The All. As Empedocles (c.495-435 BCE) says, the Divine All (To Theion Pan)
is Mind, both Holy and Ineffable,
through the whole Cosmos darting with swift thoughts. (fr. 134)
Again, the Divine Mind is not separate from the physical universe, for it is the physical universe. Its thinking constitutes Divine Providence.
The wisest way to live then, according to Pythagoreans, is in Harmony with Providence; we are a part of It, and so to live otherwise is to fight against ourselves. Wisdom is the means by which we attune ourselves to Providence. That is, "Wisdom is one thing: to know the Thought, how all things are steered through all things" (Heraclitus, fr. 41). It is a lifelong process of Individuation (Holôsis -- making oneself whole), including Unification with Divinity (Henôsis -- making oneself one), which eventuates in Deification (Theôsis).
How can we come to know The One, which is by definition ineffable, unspeakable, paradoxical, and contradictory? According to the Pythagorean Tradition, there is a Threefold Way (Triodos).
The first way is Analogy (Analogia): we cannot say what The One is, but we can say what It is like. For example, we can say that The One is like the Sun illuminating all things while remaining undiminished, and that The One illuminates each level of being in its own way, as each thing reflects or absorbs the Sun's rays in its own way.
The second path to The One is by way of Negation (Apophasis). Since The One is "beyond being," we cannot say what It is; therefore we must resign ourselves to saying what It is not. This is a Spiritual Exercise (Askêsis) that considers each assertion we might make of The One and then rejects it. Eventually one reaches a state of Silence (Sigê), awestruck and dumb before The Unspeakable (To Arrhêton).
The third way is the Ascent (Anagogê), by which we ascend to union with The All. The other two ways are intellectual, and so they must ultimately fail to achieve full understanding and illumination. It is a tenet of Pythagorean philosophy that the only way to truly know something is to become it: like knows like. Therefore the only way to know God is to become God. This may sound presumptuous, at very least! But remember that The All is God, therefore we are already part of Divinity, and thus divine ourselves. What we are seeking then is to become conscious -- intuitively, not intellectually -- of ourselves as aspects of Deity, and to live accordingly, as organs of divine Providence. An ancient Pythagorean said, "People are akin to the Gods... hence also God exercises Providence over us" (Diogenes Laertius, VIII.27). (See Part V on the Ascent.)
In order to know The One, you must become One, and so you must unify the opposites in yourself. In part this is accomplished by becoming more aware of symbols, for they can have multiple, contradictory meanings, and they are the language of the collective unconscious, by which we may communicate with the Gods.
The foregoing description of The One may seem to verge on Pagan monotheism; indeed, mystical Christianity, Judaism, and Islam all borrowed generously from the Neo-Platonic branch of the Pythagorean tradition. However, there are significant differences, which are important to observe. First, as discussed in the earlier parts of this Summary, Pythagoreans recognize many Gods; in addition to Aiôn (The One), there are Kronos and Rhea (the Monad and Dyad), Zeus and Hera (the Demiurge and World Soul), and all the other Olympians, as well as a host of lesser Divinities, Spirits, and Heroes (deified ancestors). All these Gods are aspects of The One, of course, who is sometimes called simply "God," but by this same reasoning we mortals are also aspects of God, as is everything else in the world. What this shows is that there is a false dichotomy between monotheism and polytheism. Just as we all are parts of The Whole, but nevertheless separate individuals, so also all the Gods are really distinct and individual Divinities, but also parts of the Divine One. Like our fingers, They are separate, but parts of one Whole. The All is One (Hen To Pan).
Finally, it's worthwhile to make a few remarks on the ancient Greek word Theos (God). First, in the oldest stratum of the language Theos may refer to either a God or a Goddess (the separate word Thea for "Goddess" was a later innovation). Second, the word is used almost like an adjective meaning Divinity, and so when something is called Theos, it means that it has the property of being Divine. This helps explain why an ancient Pagan may refer to Theoi ("Gods") in one line and to Theos ("God") in the next (as in the quote from Diogenes Laertius above); what he really means is "Divine Beings" (Theoi) and Divinity (Theos). The difference is between focusing on the individual aspects of Divinity or on Divinity as a whole. (It may also be worth mentioning that ancient Greek did not distinguish small and capital letters, so "god" and "God" are the same.)
In conclusion I will observe that the Pythagorean doctrine of The All reveals the full meaning of the familiar blessing:
Thou art God! (Theos ei!)