Age of Terra
"Time in its irresistible and ceaseless flow carries along on its flood all created things and drowns them in the depths of obscurity."
- – Alexiad, Anna Komnene, quoting Sophocles' Ajax
The Age of Terra is NOW! It is the age you, lovely readers all, are currently living in so you too technically are part of the history of the Warhammer 40,000 universe and part of the reason why the galaxy eventually goes down into its deep grimdarkness. Google, Facebook, and all the rest are doing their part to bring us to the next stage, the Dark Age of Technology.
We'll let you absorb that for a moment.
In all seriousness, the Age of Terra is the period in the 40k timeline from year 1 AD to year 15,001 (so a long time). It is the age when humanity moved from primitive origins to a state where they had politically unified, developed sublight drives and were spreading out from the solar system to colonise the galaxy.
Almost nothing is known by the Imperium of these early times and the pre-1 AD period where humans went from weak monkey to apex species on Earth (Jesus is mentioned in the fluff as an 'ancient religious leader' but no details) and barely nothing survives of that period. So the idea of preserving things is obviously heresy (in the Imperium yes as anything that contradicts official records is BURNED!). However that's more of a post-Heresy thing. During the Great Crusade Malcador had a museum of Age of Terra artifacts and many of the Horus Heresy books make passing reference to things from that era although in somewhat corrupted form. While Christian (known in 40k as Catheric) teachings were definitely actively supressed by Big E (along with every other religion; a character in Legion by Dan Abnett, set just on the eve of the Horus Heresy, has a character who is "Catheric by devotion," as is Ollanius Pius), snippets of plays and philosophy at least survived enough to be referenced in the 30k era. (in Prospero Burns, the remembrancers are confirmed to have 3 of Shakespeare's plays, although such is their misguided approach to information retrieval that they assume the 3 they found must have been all of them).
What information that does exist from that time period is badly corrupted into myth. For example, they believe that Luna was first reached by "The Seventeen Apollonians" and among their leaders was "Gul du Lac" who gave a parable of "Three Ursines" to explain planet habitability. It should be noted that this was recalled by Cawl who was around in M31.
Considering all that happens, it is perhaps best for everyone reading this to stay in the current time. At least we don't have Chaos Gods to worry about. Yet....
According to Fear To Tread, some kind of apocalyptic shit happened before M7.