The Cappadocian Fathers (Basil the Great, Gregory of Nyssa, Gregory of Nazianzus) worked through several issues of mysticism in the context of working out Christian theology itself, particularly in connection with heresies like the Arian heresy. The Arians were subordinationists -- they believed that God the Father was qualitatively superior to the Son or the Spirit. The Son was a creation, although an exalted one. So, the answer to the nature of the Trinity is that there is no real paradox. The Orthodox position was that the Son was of one substance (homoousios) with the Father.
Gregory of Nyssa is the most important of the three for our purposes. He develops an Athanasian, and particularly, Nicene type of mysticism. By Nicene I mean it is rooted in the Council of Nicaea (325). This was the place where the Orthodox and the Arians went different directions over the nature of the Trinity. They agreed, however, on creation, that it was ex nihilo. This doctrine was unknown to pagan philosophy, and had a very uncertain beginning in the Christian world. This doctrine means that there is a complete contrast between God and the created order. There is no intermediate zone. This agreement between orthodoxy and Arianism was what caused the problem. How does God relate to the world? Arius said that the Word is part of the world; the Orthodox said it was divine. It is important to see that this is a crisis brought on by Platonism.
Creation ex nihilo had direct implications for mysticism as well, in that it raised questions about the Platonist pattern of emanation and return. What is contemplation in all this? For the Platonic tradition, there is a sense of the soul's kinship with the divine. Now, this is not possible. Formerly, the fundamental distinction was between soul and matter. Now, it is between God and creation, and both soul and matter belong to both realms. So, the kinship of the human soul is with the human body, not with divine soul. Augustine's answer (as we shall see later) is grace.
Back to Gregory of Nyssa. He holds to this Nicene version of the world, particularly the notion of creation ex nihilo. In fact, he strengthens the distinction by using the Platonic distinction between the intelligible and the sensible. For Gregory the intelligible is divided into the uncreated (Trinity) and created (human souls), and there is no way of getting from one to the other. So, there can be no ecstacy.
This seems to destroy the very possibility of mysticism. But Gregory instead focusses on the heart of mysticism, the experience of immediacy with God himself in love. The only experience the soul can have of God is that which God makes possible. God is, otherwise, completely unknowable, for there is no point of contact. The soul, in coming into contact with God, enters deeper and deeper darkness, and knows God in a way that surpasses knowledge. This soul seeking God is one of obscure but certain presence, of being possessed.
It means that there is a different role for contemplation for Gregory. Contemplation is not the goal of the ascent. Contemplation is important in the process of purification, but things get murkier thereafter. In fact, Gregory speaks of the soul's successive entry into light, cloud, and darkness. The light clears away error, and the successive stages bring the soul into the realm of God. The Way of the Cloud is the stage in which the soul learns the vanity of created things, and also learns to see in them the manifestation of God. It is this stage that is equivalent to the contemplation of Platonic forms. But it is not just the contemplation of the principles that lie behind the world of God's creation, but also contemplation of the Word through which the world was created, both as creator and as incarnate.
Finally, there is entry into darkness. The image is Moses' entry into the dark cloud (Exodus 24:15). The soul becomes aware of the incomprehensibility of God. The soul does not reach any ecstacy in this, but just continues to penetrate the darkness. And yet, Gregory uses the simile of a mirror. Of course, there is no reflection of God, but the mirror enables the soul to contemplate by possessing in itself in a created mode what God is in an uncreated mode: it makes possible real participation in God, even while God remains incomprehensible. This results in "spiritual senses". In the darkness, the soul cannot see, but it can feel the Word. Gregory is concerned with senses that give an idea of presence -- smell, taste, and touch.
The important thing is that the soul is continually called out of itself in love for God. There is no final resting state, no ecstacy, no theoria that gives perfect knowledge. This is a very important doctrine in the history of mysticism. It means that mysticism is not a matter of intellect, but of love and will. This will influence Pseudo-Dionysius, among others. It is from Nyssa that we get notions such as the "Dark Night of the Soul", the "Cloud of Unknowing", the "Living Flame of Love", etc.
This new tradition of mysticism comes about because Nyssa tampered with the traditional Greek equation between Being and Intelligibility. It may be that being is in some sense equated with intelligibility, but not necessarily intelligibility to us (this is intelligibility quod nos, or "with respect to us"). It is not clear that God's being and intelligibility are distinct absolutely. Later Aristoteleans will make much out of this distinction.
Like Origen, Nyssa accepts the return of everything to God. Therefore, there is no hell. As well, he accepts that evil is a result of human free will, not matter. Man is the "demiurge of evil" -- we instantiate evil.
One result of this is that he believes that matter is composed of intelligible qualities. Note that this deviates from many Greeks, especially Aristoteleans. He seems to have held this so that there could be a mystical return of bodies, not just souls. This could also be associated with his work on the importance of the senses.