Considering that the word bearers were zealous beleivers in the emperors divinity before they converted to chaos. It would seem a likely twist of irony if it was then that laid the foundations of what would later become the imperial faith. Worlds they conquered had cathedrals raised to worship the emperor and basically became the first cardinal worlds so to speak. It wouldnt surprise me if opheilia was a world bought into compliance by pre heresy word bearers too. Imagine there being great temples and churches still standing in 40k that the Word bearers had actually built before their fall. Ones that maybe even Sororitas worship in....lool.