Clement of Alexandria is the first Christian writer to give a full treatment to ideas of vision, divinization, and union, which are central to mystical thought. He introduced the word "mystical" to Christian literature. His influence, though, does not extend directly to later Christian thought, but is directly felt by Origen.
Clement recognizes the Gnostic tendency, when talking about the inner reality of the soul, to regard the chosen few as having a kind of divinity which is lost in taking on flesh and found again in resurrection. He resists this, because it implies that we are inherently divine. He restates the inner life as one in which we have the potentiality for divinity which becomes actual when the soul is united in faith with the greater Christian community.
This process of divinization is new to Christian circles, and somewhat heterodox. "I say, the Logos of God became man so that you may learn from man how man may become God". But this does not mean that we all become God. It means that we are perfectable. Clement uses "theos" without the article to indicate that we become perfected while still moving around in the flesh.
Despite his problems with Gnostics, there is some sympathy for the Gnostic position. He sometimes talks about a church within the church. "Gnosis", after all, can easily be mystical knowledge. It is esoteric, for one thing. The true Gnostic is one who has attained the vision of God. This vision is not mystical identity, but mystical union; in other words, the person does not become indistinct. He also takes on a Stoic sense of "apatheia" -- the apathetic Gnostic. This person is not without feelings, but rather is the one whose passions are in control. This person has serenity of purpose. This apathetic person is both active and contemplative.
Clement also altered the Greek distinction between theoria and praxis (literally, "watching" and "doing"). Despite Plato's ideal of the philosopher-king, most Greeks saw no relationship between the two. Clement refused to reject praxis in advocating contemplation. Christian love (agape) must come out of contemplation.