For all effects, the IoM is the emperor. His orders were and are still absolute (if you claim you hear his voice, then you're given high positions instead of being thrown in the loony house).
And it's not like you can just say "Hey, the Emperor told me to say this, so you CANNOT question it." There are entire branches within both the Inquisition AND the Ministorum devoted to verifying such things, and exterminating any who are discovered to lie. This is a wobbly thing, of course, since there is little way of proving or disproving faith safe for obvious miracles (which DO happen, but even then, they're questioned, just like in modern religions).