Early Christians understood Plato well because they were Platonists. All educated citizens of the Roman Empire, especially in the East, spoke Greek and were familiar with Platonism. St Paul himself spoke Greek and was conversant with Greek philosophy.

It was the Platonist belief in the One that led Pagan intellectuals to Christianity, as stated by St Augustine (who had read Victorinus’ Latin translations of Plotinus and Porphyry).

Even though they embraced Christianity, Platonists remained Platonists at heart. When Synesius of Cyrene, originally a Platonist, was made bishop in 411 AD, he asked to be replaced by someone else because as a bishop he couldn’t find the time to practice contemplation as required by his Platonist beliefs.

Platonism did not, and could not, disappear, as there was nothing comparable in the whole Roman Empire to replace it. Instead, it persisted among the intellectual classes and was largely adopted by the upper echelons of the Church itself. Over the centuries that followed, however, Platonism became more and more Christianized and most Christians, especially in the Catholic and later Protestant West, ended up with a poor (if any) grasp of Plato’s teachings.

This is why, personally, I would recommend turning to Platonists and scholars of Plato for a better understanding. Gersons's From Plato to Platonism is a good start. I don't agree with everything he says - just as I don't agree with everything Platonists like Plotinus and Proclus say - but I think he understands the basics of what Plato and Platonism are about and can put readers of Plato on the right track. After that, they can work out the details themselves as they think best within the general Platonic framework.

It can be seen from Plato’s written works that his philosophy acquires an increasingly higher degree of sophistication over time. Plato’s Theory of Forms, for example, starts with the Meno and Phaedo where Forms are described as entities that exist as “themselves in themselves”, i.e., that are separate from the material world and from one another, and moves towards a description of Forms as blended with others and connected with the material world through copies of themselves.

Beauty and Good are not identical in every respect but they are closely interconnected, especially on higher levels of experience, with consciousness and experience becoming increasingly unified. In the Philebus, the Good is described as a mixture of three Forms, Beauty, Proportion, and Truth, and Beauty and Good appear together in other dialogues.

The combination and (partial) identification of Beauty with Good is particularly obvious in the Symposium.

To begin with, the dialogue takes place at the house of the “Good and Beautiful” Agathon. Beauty and Good are combined in Agathon himself, the party host, who is said to be “beautiful” and whose name means “good”. This could not have escaped Plato readers even under Roman rule when all educated citizens, including Christians, spoke Greek. Moreover, Socrates himself calls Agathon “very beautiful and of good nature and breeding” in the Protagoras (315d-e).

So, there can be no doubt that we are in the realm of the Good and Beautiful from the start. Socrates himself is dressed in beautiful clothes for the occasion.

Moreover, the Symposium consists of speeches dedicated to the God Eros. And, as Socrates states, Eros is the son of Aphrodite, the Goddess of Beauty and Love, and he is always “scheming for all that is beautiful and good” (Symp. 203d).

The best, most important, and most beautiful speeches in the Symposium are those of Agathon and Socrates - they are placed at the center of the dialogue and their authors are crowned by Alcibiades who has appointed himself judge over the contest.

Agathon and Socrates mirror each other in many ways. Agathon is young and beautiful, Socrates is older and not very good-looking. Agathon is a playwright who composes speeches for public consumption. Socrates is a philosopher who makes speeches addressed to small private groups. Their close connection is emphasized by the fact that they both are expressly dressed in beautiful attire for the party and they are seated together on the same couch: Agathon the Beautiful and Socrates the Good.

In particular, both value wisdom and expert knowledge above common opinion. Both view love of beauty and goodness as arising from a lack of these. And both agree that, in addition to beauty and goodness, what love lacks is truth – hence they both criticize poets for neglecting truth.

The triad of Beauty, Goodness, and Truth is an important one in Plato. All three appear together in the Phaedrus and now in Diotima’s Love Lesson. This should not be ignored.

Crucially, Agathon brings together beauty and good not only in himself but also in his speech, concluding that love of beauty brings good to both Gods and men:

And who, let me ask, will gainsay that the composing of all forms of life is Love's own craft, whereby all creatures are begotten and produced? … Since this God (Eros) arose, the loving of beautiful things has brought all kinds of benefits both to Gods and to men (Symp. 197a-b).

The connection between beauty and good is made explicit by Agathon when he brings into focus the concept of love of the Beautiful as conducive to Good. He thereby prepares the ground for Socrates’ own speech, in which Socrates takes the theme to the highest level where the philosopher who has set out on the quest for Beauty has found the Good and the Good and the Beautiful combine together with Truth to form one reality.

As already stated, the process implied in the Ladder of Love is one of inner transformation of the soul which involves interiorization, elevation, concentration and unification of consciousness.

The goal of this is nothing less than deification (theosis), i.e. “assimilation to the Divine” or “becoming godlike” (homoiosis Theo) which can only happen as a result of liberation of consciousness from the human condition.

This liberation involves the extrication of consciousness from the confines of human experience revolving on sense-perception and all the mental states based on it such as imagination, opinion, emotion, and thought, and turning our attention to higher realities.

The stages of this process are clearly outlined in the Ladder of Love. The turning of attention from one beautiful body to many beautiful bodies initiates the extrication of consciousness. The consideration of beauty in customs, laws, and knowledge brings about its interiorization and elevation. And the focus on one knowledge results in its concentration and unification.

When the extrication process has been completed, it is followed by a free, spontaneous, and sudden expansion of consciousness beyond anything known or imaginable. The philosopher no longer sees one beautiful body, or any body at all, but an infinite expanse or “sea” of ever-existing beauty (pelagos tou kalou) (Symp. 210d-e).

This is the final state of release or liberation (lysis). It is a state of absolutely free intelligence which is a state of absolute happiness which is nothing but absolute freedom and fullness or completeness and satisfaction.

When intelligence is in this state, it becomes truly creative and productive of things that are beautiful, good, and true. Of course, these beautiful things can be physical babies, who will grow to be like Agathon, beautiful and good. However, Diotima emphasises the beautiful production of poets, artists, craftsmen, architects, town planners, and law-makers who, being “pregnant in the soul” from contact with Beauty and Good, make themselves immortal by giving birth to things that are more beautiful and more deathless than man:

But pregnancy of soul—for there are persons,’ she declared, ‘who in their souls still more than in their bodies conceive those things which are proper for soul to conceive and bring forth … (209a).

This productive activity of the intelligence which has found its freedom and its true self, is not caused by any lack or need but by unceasing, overflowing and therefore creative, overabundance. Love itself is completely transformed. It is no longer motivated by a desire to acquire and possess things that we do not have, but by a desire to give things that we do have.

The key to understanding the Mystery of Eros, and to understanding Plato and Platonism in general, is the understanding of the fact that Eros here stands for the totality of states and activities of volition.

Eros refers not only to humans, but to all living beings including the Gods. Divine love or desire may seem different from human love or desire. The one stems from an awareness the Divine has of its own abundance. The other stems from an awareness (or perception) of absence of abundance. But human desire is ultimately an expression of divine desire, of the will of intelligence or spirit to be itself, i.e., to be happy and free, including free from desire.

The exchange between Diotima and Socrates is as follows:

D: You hold that love is directed to what is beautiful. But why does the lover desire the beautiful?
S: The lover desires the beautiful in order to possess it.
D: But what will the lover get by possessing beautiful things?
S: This question I am unable to answer offhand.
D: Well, let’s change the object of the question. Why does the lover desire good things?
S: In order to possess them.
D: But what will the lover get by possessing good things?
S: This I can answer easily, happiness.
D: Yes, this is the ultimate answer. We have no more need to ask for what end a man wishes to be happy (204b-205a)

Happiness (eudaimonia) is the ultimate end of all human endeavor. And we don’t need to ask why we wish to be happy because we know that to be happy means to be our real self. When we are happy, we are at peace, i.e., in harmony, with ourselves and the world.

Our real self is the intelligent spirit within us (nous) whose supreme happiness consists in contemplating the Divine within itself, i.e., itself as it really is on the highest level of existence. This is the meaning of contemplation (theoria). The divine is beauty, wisdom, goodness, and all such qualities (Phaedrus 246e). Contemplation of divine qualities, for example, beauty, elevates and refines consciousness until it acquires a direct vision of Beauty itself. The Gods themselves, who are supremely happy and blessed, derive their happiness from contemplation.

Indeed, if we are happy when we possess beautiful and good things, we can easily imagine how much happier we will be when we possess not only beautiful and good things, but the Beautiful and the Good themselves, and with them, Truth itself which is, above all, the truth about our true identity.

But happiness is of little value without the awareness of being happy. Where there is happiness, there is awareness. Awareness and happiness are the highest and most fundamental principles of intelligent life. Awareness and Happiness are the properties or faculties of Supreme Intelligence, along with Will-Power, Knowledge, and Action as indicated in the Timaeus.

Therefore, the imagery of the Sea of Beauty takes us sufficiently close to Ultimate Reality for us to conceptually grasp Plato’s Two Causes.

Though Beauty itself belongs to Ultimate Reality, the philosopher can have awareness of it because it is also within him and because his consciousness has sufficiently expanded to contain Beauty, at least partly, within its field of awareness.

These two elements of experience, (1) the Sea (or Ocean) of Infinite and Eternal Beauty, and (2) Awareness of it, are the objective and subjective aspects of consciousness, respectively, that correspond to the Dyad and the One. Awareness also corresponds to the One through its function of unifying experience.

The philosopher’s expanded consciousness and Ultimate Reality mirror one another. The philosopher arrives at Infinite Beauty and Awareness of it by a process of ascent or return (epistrophe) to the Ultimate Source and Cause of all. In contrast, Ultimate Reality arrives at the stage of the One and the Dyad by a process of descent or procession (proodos) from the Ultimate Source and Cause of all.

The stages of Consciousness prior to Creation are as follows:

1. Pure, Undivided Intelligence or Awareness (syneseis or synaesthesis) a.k.a. “the One”.
2. Self-Aware Intelligence, i.e., Intelligence with Consciousness (parakolouthesis) or Self-Awareness (parakolouthesis heauto) = Intelligence (subjective element) aware of itself (objective element) = “Indefinite Dyad”
3. Creative Intelligence (nous poietikos) = “Creator-God” = Intelligence containing Forms = Knowledge

Otherwise formulated:

(A). The good is defined by beauty (kallos), proportion (symmetria), and truth (aletheia) (Phileb. 65e).
(B). These properties depend on order which is a well-proportioned arrangement of parts in a harmonious whole.
(C). Therefore, the basis of order is unity or oneness.
(D). Therefore, Unity or Oneness is the cause of all good.
(E). But Order and Goodness in the world are not perfect.
(F). Therefore, a cause must exist that is opposed to Oneness and Goodness.
(G). Such a cause must be a principle of Division and Plurality.
(H). This cause is the Indefinite Dyad.
(I). Therefore, there are two causes, the One, and the Indefinite Dyad.
(J). But the Indefinite Dyad exists exclusively in opposition to the One.
(K). Therefore, the Indefinite Dyad is dependent on the One.
( L). Therefore, the One (= the Good) is the Ultimate Cause of all.

When the One, i.e., Supreme Intelligence, sets about to create the Universe, it limits its own powers by imposing Limit on the Unlimited, and thus produces (1) Spirit or Soul which possesses exactly the same powers as the Supreme but in limited degree and (2) Matter which is (almost) completely devoid of intelligence.

Were this not the case, the human soul would not have the powers of awareness, happiness, will, knowledge and action, and would be no better than inanimate objects. Indeed, it would be worse given that even inanimate matter, though devoid of higher intelligence, still possesses some powers as can be seen from the behavior of atomic particles, energy fields, etc. – which, at the very least, indicates the presence of a very limited power of action.

It follows that human love or desire for the beautiful and the good, and ultimately, for happiness, is really an expression of divine will, i.e., of the will of limited, individual intelligence to recover its original happiness and freedom which it once had before descending into particular existence.

This act of volition (boulesis) on the part of the human soul is triggered by the perception of beauty in objects other than itself.

The perception of objective beauty activates the soul’s innate memory of the “infinite Sea of Beauty” that was once part of its self-identity, and, through philosophic practice the soul gradually recovers its full awareness of its true identity. Having recovered its identity, it is once again complete, fully satisfied, self-sufficient, self-contained, full of Beauty, Goodness, and Truth, infinitely and eternally happy, and lacking in absolutely nothing. It has now attained ultimate perfection (and is welcomed into the company of Gods).