Orthodox Platonism

The apophatic theology, whose roots are found in Philo and Clement of
Alexandria, was specially developed by Gregory of Nyssa and Denys the
Areopagite.

[5] Disc.7,21,2-33; SC 405,232-236.

But there is danger for all three of them. For it is difficult to
comprehend (God), it is impossible to express (Him),[9]

[9] Cf. Plato, Timaeus 28c.

http://en.hilarion.orthodoxia.org/6_5_2

For one who is not pure to lay hold on pure things is dangerous,[1]
just as it is for weak eyes to look at the sun’s brightness.

[1] Cf. Plato, Phaedon 67b.

http://en.hilarion.orthodoxia.org/6_5_3

In this idea, one can discern the influence of Plato, who said that to
reach pure knowledge is only possible when one is liberated from one’s
body and contemplates things with one’s soul alone.[21]

[21] Plato, Phaedo 65e-66d.

http://en.hilarion.orthodoxia.org/6_5_4

The image of the sun, which derives from Plato,[8] is one of Gregory’s
favourite images when he speaks of God.[9] Gregory uses this image in
particular when he speaks of the human person’s striving to God as the
highest good:

...From many and great things... which we receive and will receive from
God, the greatest and the most generous is our inclination and our
kinship to Him. In the realm of intelligible things God is the same as
the sun in the realm of sensible.[10] The sun illumines the world
visible, while God illumines the world invisible; the former makes
bodily sights sunlike, while the latter makes intelligible natures
godlike. The sun, while giving a seer to see and a seen to be seen, is
itself incomparably more beautiful than what is seen; in the same
manner God, while giving a thinker to think and an object of thinking
to be thought of, is Himself the climax of everything intelligible,[11]
so that every desire finds its end in Him and nowhere else reaches
forward.

[8] See Republic 508c-509b.

[9] This Platonic image is also used in Disc. 21,1; 28,30; 40,5; 40,37;
44,3.

[10] Cf. Disc.28,30,1-3; SC 250,168.

[11] Plotinus, Enn.6,7,16: ‘The sun, cause of the existence of
sense-things and of their being seen, is indirectly the cause of sight,
without being either the faculty or the object: similarly this
Principle, the Good, cause of Being and Intellectual-Principle,is a
light appropriate to what is to be seen There and to their seer;
neither the Beings, nor the Intellectual-Principle, it is their source
and by the light it sheds upon both makes them object of Intellection’.

http://en.hilarion.orthodoxia.org/6_5_7

The theme of illumination is, in turn, inseparable in his thought from
the theme of purification (katharsis).[6] Gregory inherited an interest
in this theme from his studies of ancient Greek philosophy, where
katharsis is one of the key notions.[7]

[6] Cf. C. Moreschini in SC 358,62-70; T. Špidlik, Grégoire, pp. 75-83.

[7] On the idea of katharsis in Plato and Plotinus see A. Louth, The
Origins of the Christian Mystical Tradition, pp. 7-10; 44-47.

To understand this text, one should remember that in the Platonic
tradition the way to perfection was perceived as one from multiplicity
to simplicity, from duality to unity. Plotinus, in particular, claims
that in order to come to the knowledge of the Unity we must become one
from many.[12] Contemplation of the One is, according to Plotinus, a
total unity with the One which excludes all multiplicity or diversity:
‘There were not two; beholder was one with beheld; it was not a vision
compassed but a unity apprehended. The man formed by this mingling with
the Supreme... is become the Unity, nothing within him or without
inducing any diversity; no movement now, no passion, no desire, once
this ascent is achieved... It was a going forth from the self, a
simplifying, a reunification, a reach towards contact and at the same
time a repose’.[13] The highest stage of the mystical ascent is a state
of ecstasy, a total mingling with the One and diffusion in Him. The
vision of the highest Intellectual-Principle is connected in Plotinus
with the experience of the vision of light emanating from it.[14] One
can, of course, point to the disparity between the ecstasy of Plotinus
as a diffusion in the impersonal One and the mystical contemplation of
Gregory as an encounter with the personal Deity, the Trinity. Yet one
cannot but see a startling similarity of language, terminology or
imagery between the two authors.

[12] Enn.6,9,3.

[13] Enn.6,9,11. Cf. Gregory Nazianzen, Disc.21,1,25-26; SC 270,112:
‘God is the limit of everything desirable and repose from every
contemplation’.

[14] Enn.5,5,7-8.

http://en.hilarion.orthodoxia.org/6_5_8

However, he does not limit himself to scriptural and traditional
sources; he also borrows something from ancient Greek philosophy.
Echoes of Plato[3] are discernible in the following text:

I believe the words of the wise, that every fair and God-beloved soul,
when, set free from the bonds of the body,[4] it departs from here, at
once enjoys perception and contemplation of the blessings which await
it... and goes rejoicing to meet its Lord... Then, a little later, it
receives its kindred flesh... in some way known to God, who knit them
together and dissolved them, enters with it upon the inheritance of the
glory there. And, as it shared, through their close union, in its
hardships, so also it bestows upon it a portion of its joys, gathering
it up entirely into itself, and becoming with it one spirit, one
intellect and one god... Why am I faint-hearted in my hopes? Why behave
like a mere creature of a day? I await the voice of the archangel, the
last trumpet, the transformation of the heavens, the transfiguration of
the earth, the liberation of the elements, the renovation of the
universe.[5]

[3] Cf. Phaedrus 246a-256a.

[4] Cf. the Platonic image of the body as a prison for the soul: Plato,
Phaedo, 62b; Kratylus 400c.

[5] Disc.7,21,2-33; SC 405,232-236.

http://en.hilarion.orthodoxia.org/6_5_11

And finally:

esp. pp13-15).

Liturgy and Mysticism: The Experience of God in Eastern Orthodox
Christianity. Part I

http://www.marquette.edu/maqom/Liturgy.pdf

(Just scroll down for the good parts).