It is a passage from the Mystagogical Catechesis of St. Cyril of Jerusalem which dates back to approximately 347 A.D. In the catechesis on the Rites Before Baptism, Cyril writes:
However, thou art bidden with arm outstretched to say to him as though actually present "I renounce thee, Satan". I wish to say, wherefore ye stand facing to the west; for it is necessary. Since the west is the region of sensible darkness and he being darkness, has his dominion also in darkness, ye therefore, looking with a symbolical meaning towards the west, renounce that dark and gloomy potentate.
Cyril then continues by describing the meaning of the each line of the renunciation of sins. Afterwards, he writes:
When therefore thou renouncest Satan, utterly breaking all covenant with him, that ancient league with hell, there is opened to thee the paradise of God, which He planted toward the east, whence for his transgression our first father was exiled; and symbolical of this was our turning from the west to the east, the place to light. Then thou wert told to say, "I believe in the Father, and in the Son, and in the Holy Ghost, and in one Baptism of repentance."
Imagine my shock when I first read this. Cyril of Jerusalem's Mystagogical Catechesis rocked my Liturgical understanding. I still don't know if I think Mass should be celebrated ad orientum, but I will tell you that I think baptisms should.
I recommend the Mystagogical Catechesis, especially the one on baptism, of which I am most familiar. I haven't read the one on the Eucharist yet, but I am sure it is just as good. In fact, I wrote a term paper based on the Mystagogical Catechesis which compared the use of Chrism as he describes and to how it is used in the modern rite and ended up concluding the we don't use nearly enough Chrism. That wasn't the paper I set out to write, but the evidence was irrefutable.