A reader says:
You said, “an infallible definition is never new revelation” but isn’t that just a mite disingenuous? The Immaculate Conception and Assumption of Mary into heaven were both infallibly declared but both are about as close to new revelation as one could possibly get, given that there is no Scriptural support for either and little support in Tradition.
Remember: Catholics don’t think that all revelation is conveyed only in Scripture. They believe the Tradition transmits revelation as well. Both the sinlessness and the “dormition” or Assumption of Mary are ancient features of the Tradition (which is why the Orthodox, who split from Rome a thousand years before the definitions were promulgated) regard them as part of the Tradition. Neither definition added anything new. They merely defined more closely what the Church has always believed.
It is true that the Orthodox reject the Immaculate Conception, but their reason for doing so is not much consolation to your average Protestant. Protestants take such a high view of original sin that they can’t believe even Mary was exempt from it. Orthodox tend to reject the idea of original sin and therefore never bothered with the question of how Mary (whom they, like Catholics, believe to have been “Panagia” or “All Holy” or stainless) got that way. Catholics defined the dogma of the Immaculate Conception in order to clarify how a belief they shared with the Orthodox (Mary’s sinlessness) could be reconciled with a belief they share with Protestants (original sin). As usual, it offended both the Orthodox and the Protestants. Part of the genius of our Faith.
The Assumption (or Dormition as it is known in the East) was made a feast in the Eastern Church in the fourth century. This is considerably before 1950 and so is hardly “new revelation”. Feasts are like icebergs in the ancient Church. When the Church gets as far as proclaiming a feast, we are looking at something that is widely received as “common knowledge” by everybody (much like nobody would be stunned if the Church formally defined that a human person exists from conception tomorrow morning). Notably, the promulgation of the feast seems to have generated no controversy at a time when itsy bitsy doctrinal variations generated huge controversies (think “homoousious vs. homoiousious”) or the giant screamfest that erupted over “Theotokos vs. Christotokos”. This provides strong support for the contention that it was common knowledge in the early Church that Mary had been assumed into heaven. It was a non-controversial proposition to a Church that was easily given to controversy.