1. Links between the mortal and the divine...
Strictly speaking, as the "divine" is simply the "positively" focuses psychic energy of the teeming masses of the Imperium (Think Hellblazer) it is
doubtful that they were instruments of "His will" as "He" isn't willing much these days.
(Personal speculation! Warning! Warning!)However, I theorize it is entirely possible that by the combined faith of the Imperium, a warp entity was created, much like Slaanesh, and that HE, the Emperor, a true GOD and not some
superhuman-psychic-corpse-on-Terra, could will stuff. And the Saints could be sent, and directed by him.
Where, for all their psycic power, the Eldar in their depravity gave birth to Slaanesh, so by the pure, honest, loving faith of the masses was a being born, wrathful against the enemies of mankind, but willing to save (so long as the 'dex is Imperial) his servants.
All in all, humanities ultimate salvation might arise from their worship after the Emperor's demise, as what protection was there for their souls under the aethistic regime of his-imperial-greatness? Yet now, it is known that in the warp is a great light, and a place of peace and rest, where souls are protected and free from danger, free from the perils of the Warp. Why? Because enough folks with enough psychic energy coursing through them willed such a place into being. It could be said that the faithful created the saints, in a roundabout sorta way.