Why "godless empire would fall to the claws of aliens"?

Question

In the "First Heretic", Lorgar meets a demon guide Ingethel, which shows primarch the "truth" about Chaos. Ingethel also says, that "Chaos will be humanity's salvation, godless empire will fall to the claws of aliens, when humans will follow the fall of Eldar".

Was the demon lying?

The Eldar fell because their Cult of Pleasure created something horrible, something that they weren't able to accept as their god, hence Slaneesh absorbed them (body and soul) into himself. The Imperial Truth was atheistic dogma, so would it mean, that Empire would create a Chaos god of... atheism? It doesn't seem too plausible, since the Imperial Cult was already taking hold and, in the long run, it wouldn't matter would Emperor be worshipped as a god or just a immortal hero.

Or maybe there is another explanation - victorious Empire without Horus Heresy would easily conquer whole galaxy and wipe all resistance. Following that, without any threat it would start neglecting the military, leaving it open to the arrival of Tyranids some ten thousands years later?

Answer 1

I always assumed it was telling the truth

It's impossible to know if Ingethel was lying or being sincere, we're never directly told if everything it shows Lorgar is real or completely false. It could have been lying, it is a demon of Chaos after all.

I always assumed it was telling the truth.

Without Chaos, mankind might have gone on with the Great Crusade and like you say:

...victorious Empire without Horus Heresy would easily conquer whole galaxy and wipe all resistance. Following that, without any threat it would start neglecting the military, leaving it open to the arrival of Tyranids some ten thousands years later?

I personally think this is what would have happened as well.

Mankind might have been destined to have been wiped out by the Tyranids, Chaos could very well have stepped in and saved us.

But the question is, is it better we continue to exist under the influence of Chaos or is it better we are completely wiped out instead?

Ingethel told Horus the only way to save humanity was to side with Chaos against the Emperor, the Cabal told Alpharius the only way to defeat Chaos was to join Horus and fight against the Emperor, their argument was when Horus won, he would become insane and go on to wipe out all humanity, thereby defeating Chaos, without mankind there can be no Chaos.

Alpharius sided with Horus with the intention of winning the war against Terra, and consequently enabling Horus to wipe out all mankind, actually saving us from a fate worse than death, the corruption of Chaos.