What "shifts" mark the transitions between major periods of church history after the Resurrection and before the European Reformation?
The answer to this question, of course, depends on the historian / theologian one consults. However, in the broadest sense, we would all agree on most of it.
Number One - The first major shift would seem to be the transition from Judaism to Christianity that takes place on the first Pentecost following the Resurrection. This was a major shift, as Acts 2 suggests.
Number Two - The second major shift seemed to come about in Acts 10 when Cornelius, known by most people as "the first Gentile convert." The inclusion of the Gentiles and Jews together in one system was a bombshell to traditional thinking.
Number Three - The next, which seems to bleed together with number two, could be the persecution of the church that took place from Acts 8 and for the next several decades. Three different waves of persecution.
Number Four - When Constantine issued the Edict of Milan in 313 AD a major shift was experienced that would result in the eventual development of the Roman Catholic Church.
Number Five - When Luther nailed his "95 Theses" to the door of his church in 1517, the final shift contained in the question occurred.