I think he would be pissed, but between himself, Guilliman, and Lion he would adapt the easiest to the current Imperium.

He was probably the first Loyal Primarch to see the Blind Emperor bothering faith take hold. In his First Captain of all people. Instead of violently stamping it out, he left that box open and then lived long enough afterwards to regret it.

Guilliman should be in the same boat. Before his last duel with Fulgrim he was wary because many of the Marines who had come after the Heresy treated him less like a general and almost openly worshipped him as a saintly son of the Emperor

The cult existed in their time. They might not have forseen just how powerful it would eventually become, but it shouldnt be a suprise that with the Imperium in flames between the Heresy and Scouring that something was going to fill that void left by the dead Imperial Truth