When did Truth learn the Great Journey was a lie?

So, as evidenced by Halo 3 and late events in Halo 2 Truth had obviously known. For how long did he (and presumably Regret and Mercy) know?

It was actually before their ascent to power. Back then, Mercy held a title known as the Philologist, whose responsibility was to be a caretaker for a dormant Forerunner AI they called the Oracle. Truth and Regret (Then the Minister of Fortitude and Vice Minister of Tranquility) visited the Philologist some time after the Battle of Harvest, and the Oracle miraculously activated for the first time in thousands of years. It told the future High Prophets that the Covenant had misinterpreted some Forerunner glyphs, and as a result, the entire basis of the Covenant’s religion was a lie. The three then disconnected the Oracle from the Forerunner Dreadnought to prevent the truth from being revealed, they rose to power, and we all know the rest.

Actually, the High Prophets never believed the Great Journey (i.e. transcending to godhood through the Halo array) was fake. The confusion about the Great Journey was around how humans were related to it. Originally, the Covenant mistook the glyph for “Reclaimer” as “Reclamation”. Then the Prophets who would become Truth, Mercy, and Regret learned from Mendicant Bias that they had translated the glyph wrong. Truth, Mercy, and Regret realized then that humans were the true inheritors of Forerunner legacy, or that they were surviving Forerunners. Either way, they starting thinking that the Great Journey would not grant godhood to all who walked the righteous path. So in order to snag power and keep the Covenant under control, they decided to lie and continue telling the Covenant that all of them would go on the Great Journey, even though they knew themselves that not everyone in the Covenant would become gods. However, the Prophets still truly thought that they themselves would transcend. Truth died actually thinking that the Great Journey would have made him divine. The Prophets were misinformed and selfish, but not crazy; they weren’t purposefully trying to destroy all life in the galaxy with the Halos. The truly believed that the Forerunners were gods and that the Halos would have made them gods, too.

Man, just to imagine the look on Truths face if he lived long enough to witness a god IRL (Hence the Didact) Just thinking of that makes me wonder how the Covenant would’ve been if they found him during the Human-Covenant War.