What parameters should be set for an unholy Trinity to be worshipped as one bein

Zoth-Ommog is an ancient and demonic god that exists outside reality and seeks to enter our world. To accomplish this, he took a virgin to be his "bride" in order to reincarnate a portion of himself into the mortal realm. This mortal would be inseminated with a piece of Zoth's power in order to produce a child.This prophecy would eventually break down the barriers between realities that would eventually lead to both realms becoming one.

This dark Messianic figure would go on to begin the process of combining both worlds, but would eventually leave this reality, leaving the unfinished work to his followers. The religion that developed around this god is well aware of this truth, and how this prophecy involved three individuals: Zoth, the mortal bride, and the product of their union. However, instead of worshipping these three beings as separate gods, I would like this faith to go on to worship this trinity as one being, similar to the blessed trinity in Christendom. How can I ensure that this outcome is possible?

 

The problem is that I can't find a way for this to make sense practically. If the son is supposed to be divine, then the mother should be divine as well, as she is also representing the creator. However, just like Mary in the story, she is simply a mortal woman with no godhood attached to her. The logical solution would be to give her some kind of divinity, but if an aspect of God is already on earth in mortal form, that defeats the purpose of having a son in the first place.