God is One. There cannot be many Gods for the following reason. Suppose a theological belief that there are several Gods. The first God is not only perfect, but infinitely perfect, lacking in absolutely nothing, and existing unbounded by time and place. So what would a second God be like? He could not be the same as the first, but in a different place, or a different time, for any God by His Nature must be unlimited by time and place. A second God cannot be distinguished from the first God by being more perfect, or less perfect. A second God cannot be distinguished from the first God by having different qualities, since any God must be infinitely perfect in all that is Good; He must be Goodness Itself. There is nothing left for the Nature of the second God. So not only can there not be many Gods, there cannot be more than one God.
To explain this again by an analogy: suppose that there are several very wealthy men on earth, and to simplify, let’s say that wealth is measured only by ownership of land. There could be many very wealthy men, each owning vast expanses of land. But suppose we instead postulate several wealthy men, each of which is perfect and complete in all wealth. The first wealthy man will own the entire earth (since we are measuring wealth in this example by ownership of land). But there will be no more land left for a second man to be perfect and complete in wealth. So there can be only one.
Similarly, God is perfect and complete and infinite in His Nature; He is infinite Goodness. So there is nothing left for a second God to be.
Thus, the Three Persons of the One God do not each have His own perfect infinite Nature; the Nature of God is One. And the Nature is not shared in the sense of each Person possessing only part — each possesses the entire Whole of the Nature, without any detriment or loss or compromise to the possession of the other Persons of the same Nature.
But this poses a problem. What distinguishes the three Persons? There cannot be more than one infinitely perfect God, so the Nature is One. And God is ‘truly and absolutely simple’ (as both Augustine and Aquinas taught). So the one Nature is not divided into three parts. The Three Persons are distinct, yet each is perfect God, having the one and same Divine Nature.
The only basis for distinction between the Persons is procession.
The Father does not proceed.
The Son proceeds only from the Father.
The Spirit proceeds from the Father and the Son.
This distinction allows there to be Three Persons who are truly distinct from one another.
But if the Spirit proceeds only from the Father, just as the Son does, then there would be no distinction at all between Son and Spirit. For each is infinitely perfect God.