Vatican II did not "require’ the change of the priest from FACING the Altar to
Facing the People. That was done by people’s own voluntary decision without usually asking the Holy See’s permission and became WIDESPREAD and accepted.
There is no “sin” in it, but the Old way did have a valid reason behind it:
Historically, Christians faced EAST when praying the Divine Liturgy, the Mass.
The altar was at the east end of the church, and the priest, leading the people,
faced EAST, toward the altar, during most parts of the Mass.
sacredarchitecture.org/articles/the_eschatological_dimension_of_church_architecture/
Actually, that is only half of the story. The first churches were all built with the altar in the west and the mass was celebrated versus populum. This was based on Jewish synagogue architecture of the time, in which the sanctuary faced east (or jerusalem, depending on the tradition in question). In the masses, the people would face the priest, and then for the consecration they would turn and face the east.
In time (circa the 6th - 8th century), churches were constructed with the altar in the west, with the justification that this was a symbol of unity and “the priesthood of the faithful”. Logistically, it might have been easier for the priest to turn around and face east than for the entire congregation.
The notion that the priest was symbolically “leading the people to God” is a romantic one, but it is also false… all written justifications of the change seem to point to the idea of unity and emphasizing the priesthood of all the faithful, not the priest as the sole leader at the head of the crowd. The people faced east because that was the traditional view of where Jesus was supposed to return from, that was it… and anyone familiar with Jewish theology knows that you don’t approach Elijah in the east, but rather that celebrations are held in the view that Elijah will approach YOU from the East… putting even more to rest the idea that the mass somehow represents movement with the priest at the front.
Personally, I prefer the priest facing the people, not because WE are the focus of worship, lord no, but because I can hear him better when he is reciting the prayers of the mass, and I can see the consecration occuring much more clearly etc. I’ve been to the Latin mass, which is BEAUTIFUL, where the priest faces the East, and during the consecration he has to hold the host WAAAAY up in the air for the congregation to be able to offer an act of adoration and be able to see it.
For those reasons, I prefer the new way. But the Church never “required” this.
The fact that it is not REQUIRED is certainly correct, and I agree with you that I appreciate being faced by the priest because I can hear and see the consecration going on. At the same time, I would point out that there is an associated downside to this: that priests sometimes trip on their own tongues, or forget a minor detail here and there. When the priest faces away from the altar, the people cannot see this, and rarely can they hear him speaking… so the focus is rightly on the awesomeness of the consecration. When the priest FACES the people, all his actions and words are exposed, and I fear that some people occasionally use this as an opportunity to play a game of “gotcha” with the priest and scream abuse over very minor errors or slips of the tongue.
Is either way BETTER? No, not really. Both changes (to ad orientem and back to versus populum) were institutied using the same logic… so it is likely that the associated value has less to do with some objective “right way” and more to do with contemporary attitudes towards what constitutes an inclusive unity and universal priesthood (originally that we all needed to face east to be universally included and now that we all face the altar to be universally included). There are trade offs either way.