This is what the Codex says:

There was a time when the sons of Ferrus Manus held metal as superior to flesh, but in recent decades these masters of mechanised warfare have balanced clinical precision with the insights of a righteous soul.



If Ferrus Manus hadn't died on Istvaan the would have guided his Legion away from their belief in 'the flesh is weak', and also removed the metal covering his arms. It seems something has altered the Chapter's beliefs, but what was it?



They don't believe the Emperor is a God (the purity of soul bit was let slip by my GW store manager when I bought the Codex, a hint of what's to come).