The Witness of Archaeology
It has been proved beyond any shadow of doubt that from the time the Christians were first allowed to build churches, they constructed them along an east-west axis. Some had the apse containing the altar at the east end, and the entrance at the west; in others this procedure was reversed. By the end of the fourth century almost every church building in the East had the apse at the east end, and by the second half of the fifth century, this was also the case in the West. But, even where the altar was situated at the west end of the church, Mass was still celebrated facing the East, and so the altars of these churches were constructed in such a manner that the priest could stand on the west side in order to celebrate Mass facing the East. This arrangement can still be seen in such basilicas as St. Peter's in Rome, and it is precisely this arrangement that has given rise to the myth of Mass facing the people as a practice of the early Church.
But surely, it might be argued, if the celebrant stood on the west side of the altar, as he would have done in St. Peter's Basilica, then Mass must have been celebrated facing the people. Not at all. The ancient practice in churches where the apse was at the west end was as follows: the congregation did not stand directly in front of the altar but on either side of the nave, the men on one side and the women on the other. During the first part of the Mass, the Mass of the Catechumens, the congregation would face the celebrant in order to hear the readings and the homily. But for the Mass of the Faithful, now known as the Eucharistic liturgy, they would all turn to face the East. Ancient liturgies contain directions for the congregation to face the East, or, as the instruction was usually expressed, to turn toward the Lord (conversi ad Dominum). Turning toward the Lord, symbolized by the rising sun, and turning toward the East, were synonymous. This expression is found at the conclusion of forty-seven authentic sermons of St. Augustine. The construction of the altar in such basilicas as St. Peter's was, then, to make possible a Mass facing the East and not a Mass facing the people. The Missal of St. Pius V contained a rubric which had been drafted to cater for the situation in the few Roman basilicas where the altar was at the west end. This rubric referred to altars sited versus populum, facing the people. Long before the pontificate of Pope St. Pius V, the practice of siting the altar at the west end of the church had become virtually non-existent outside a handful of Roman basilicas. The custom of turning to face the East had naturally died out. But, not surprisingly, the practice in a few important basilicas was catered for in the rubrics of the Roman Missal, and, as the priest now celebrated facing the people in them, it was only natural that the phrase versus populum was used. 5 It is possible, probable in fact, that those who drafted the rubric were unaware of the reason for the deviation from normal practice found in these basilicas. But with the findings of modern scholarship no one purporting to be an authority on the liturgy has the least excuse for claiming that the siting of an altar in a few basilicas in Rome, and a rubric in the Missal of St. Pius V, prove that Mass facing the people was the ancient custom of the Church.
5. See Ritus servandus in celebratione Missae, V. Article 3.
Emphasis in bold, that of the Web Master.