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 it's deep grimdarkness.
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 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 (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, 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) .
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.