Ad Orientem
The first time I saw the Mass celebrated Ad Orientem (the priest facing East) was at an Anglican Church. To be fair, I don't remember my exact thoughts, but it was probably something like, "What is he doing?!" To many people, it might seem like a Roman practice that is caught up in a nostalgia of bygone eras. To be fair there are probably many Roman Catholics who don't even know why some of their churches do it. In fact, in the Roman Church, the Mass used to be only celebrated Ad Orientem, until Vatican 2, which allowed for the priest to celebrate facing the people (Versus Populum). Almost overnight many of the Roman churches moved the altar away from the eastern wall and celebrated facing the people. Now the majority of Roman Churches that celebrate Ad Orientem are considered Traditional Catholics who often want the old Tridentine Mass.
In the Reformation, many Continental Reformers argued against Ad Orientem because it was tied up in the idea of the priest functioning in a High Priestly role where he was the one who was offering a sacrifice, essentially as the One Mediator and Advocate. Additionally, some have since claimed that it hinders people from being able to see and hear what is going on. However, many have noted that clericalism exists in Versus Populum too, with the presider seeming like he is the true Host of the Lord's Supper. So why has the Church continued this practice, even when challenged?
Beginning with Solomon, the Jews were told to face the temple when they prayed. This is shown in Solomon's prayer: "When heaven is shut up and there is no rain because they have sinned against you if they pray toward this place and acknowledge your name and turn from their sin when you afflict them."(v. 35). Orienting yourself when you pray has Jewish roots and continues even to this day for Jews who face Jerusalem when they pray.
So why do Christians face the East? Just as Jews were to face the temple, for Christians, Christ is the Temple, the place where God dwells. The Early Church Christ saw the reference "The rising Sun of Righteousness" in the prophet Malachi as a reference to Christ: "But unto you that fear my name shall the Sun of righteousness arise with healing in his wings; and ye shall go forth, and grow up as calves of the stall." (Malachi 4:2) They also noted that not only does he come from the East but this is where he shall return: "For just as the lightning comes from the East and flashes even to the West, so will the coming of the Son of Man be." (Matthew 24:27) The theological reasons for praying Eastward goes back to the Old Testament and the Early Church Father's understanding of Scripture. Praying Eastward goes back to the beginning and is considered one of the oldest unwritten laws of the Church.
These theological reasons also have practical implications. When we talk to God in worship, we symbolically (since not all churches face East) and liturgically turn toward God in prayer. When we pray together, we pray in the same direction. Just like when you are in the presence of a King you wouldn't turn your back on them, you are supposed to back out of the room as a sign of respect. So, in worship, we face East, especially at the Gloria Patri, Gloria in excelsis deo, and the creeds. When you see those serving at the altar turn, we aren't facing the cross or altar but the East.
An interesting note is that in funerals the casket is faced with the person's feet facing East, symbolizing that when they rise they shall see Christ coming. The only exception is for a funeral for a priest, their head faces East, so when they rise they shall see the Return of Christ in the eyes of those they have ministered.
Ultimately, we face East because this is what worship is all about. We enter into the presence of God together and worship him. This is the priest's task, to lead people into the presence of God who faithfully and graciously gives Himself to us. And all of our worship points us to live lives of eager expectation for Christ's coming again.
Maranatha,
Fr. Aaron