I heard somewhere that the practice of reading the Johannine Prologue after the Mass (Gregorian Rite, obviously) was an introduction from the East. Can anyone care to elaborate on this?
Not that I’ve ever heard. The one thing that I have heard concerning the East/Orient is that the Armenians have (or had) a similar custom. I’ve never run across any reference to it in any other Eastern or Oriental tradition.
It’s quite clear from the structure of even the EF that it was “tacked-on” so-to-speak, and is really not part of the Mass as such. To my knowledge, in the West the Last Gospel (aka Johanine Prologue) was originally said by the celebrant as a recessional prayer. I don’t have the Pontificale Romanum handy, but I believe prior editions (meaning pre-conciliar), contain references to that.
The Armenians have the custom because it was borrowed from the Latins. That why they have some similar vestments, Psalm 42, a Confiteor, borrowings in the ordination rites, etc., etc.
The Armenian Liturgy does have the Last Gospel. Here’s the full text of the Liturgy armeniapedia.org/index.php?title=Armenian_Church_Divine_Liturgy