...there were two models for Christian church buildings: the Temple and the synagogue. The sanctuaries of the earliest post-Constantinian churches in Christendom, most prominently St. Peter's in Rome, imitated the westerly location of the Holy Place in the Temple.
Plan of Old St. Peter's as constructed by Constantine.
North is up and the Sanctuary is to the west.
The church buildings which imitated the synagogue, however, took a different tack. While the synagogue's bema pointed to the Temple, the church building's sanctuary now pointed east. The direction of Christian prayer was always toward the east--ad orientem. In the words of St. John of Damascus, "we worship God seeking and striving after our old fatherland," i.e., the Garden of Eden. We also face east in expectation of the Second Coming:
"For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be." (Matthew 24:27)
This was so even when the sanctuary of the church building was located to the west--the congregation simply turned around and faced east. The nice thing about placing the sanctuary to the west is that the building itself faces east, both to imitate Paradise lost (whose gate was in the east), and to join in the direction of striving.
Placing the sanctuary in the east, on the other hand, has certain advantages. For one, there is now no need for those hearing Mass to turn around, putting the altar behind them. Also, the whole movement toward the church, through the preparatory forecourt, into the nave, and toward the Sanctuary boundary for Holy Communion is now an eastward movement--the whole is now a striving for our fatherland. The movement is from darkness, the sun's setting, toward light, the sun's rising. From birth in Original Sin, to death in Sanctifying Grace.
Read the entire piece here: Parts of the Church Building: The Sanctuary
