Monarchian Trinitarianism states that if Jesus is really, truly, or properly God’s Son, this must entail the Son sharing a generic nature, genus, or species with God the Father, since this is the case in all generation that takes place within creation; men beget men, horses beget horses, and thus God must beget God. But while insisting that Jesus’ sonship to the Father must entail a shared nature because of the parallel with begetting in creation, they deny almost all other elements of generation of offspring that we see in creation, including all notions of corporeality, sexuality, and temporality.
Monarchian Trinitarians present their christology as being very simple and natural- just the plain, obvious meaning of what it means for Jesus to be God’s Son. Yet, this isn’t actually the case; really, they want to use only one aspect of human procreation, namely, that a son shares a nature/species with his father, and reject most other significant parallels, like the temporal implications that a father is prior to his offspring, or the corporeal and sexual connotations that accompany taking such language literally. This decision to say that some parallels with generation in creation must also be true in the case of God and His only-begotten Son, while freely disregarding others, seems totally arbitrary.
If the generation of the Son is literally the same as that of humans, then it must be bodily, sexual, and temporal, as well as communicate a species. But if we can say that three of those things cannot apply to God, because of how different God is from men (being incorporeal and eternal), then why not say the same for the fourth category, communication of a nature/species, as well? Monarchian Trinitarian views on the Son’s generation seem to arbitrarily pick one aspect of generation to apply to God and Jesus, while arbitrarily rejecting all others, with no scriptural or rational justification for these decisions.
One significant hidden assumption of Monarchian Trinitarianism that plays into this is the assumption that the one God has a kind, or species, like creatures do, which may be passed on to an offspring. But this assumption appears to have no greater warrant than assuming that God is corporeal, which Monarchian Trinitarians rightly reject. This hidden assumption that God has a nature He passes on to others should not be taken for granted, as it runs directly contrary to the scriptural revelation that God is incomparably greater than all; His greatness is not merely that he is, like Adam to mankind, the first of a given species, but He is rather said to have none like Him, none with whom He can be compared. This must certainly prove false if God has a Son which is essentiality identical to Him, the way Seth was to Adam.
But this notion that God has a species to be passed on by procreation is, at the end of the day, simply assuming that God is like men. Why would the one God, who has neither beginning nor end, and is unlike His creation, Who has no need for procreation, have a species or a nature to pass on to offspring? Certainly, a lot of philosophers and theologians have speculated that this is not the case. For instance, an important element of ‘classical’ Greco-Roman theism is that God is simple, meaning, He is uncompounded of parts and there exists no real distinction within Him between any one part of Him and another (because there are no such parts). If such a theory holds true, then it follows that God could not be divided into ‘person’ and ‘essence’, such that a distinction could exist within Him between that which constitutes His individual identity and that which constitutes His generic and sharable nature or species. Without such an internal distinction in God, eternal generation will be impossible; for without a distinction between God’s personal identity and God’s nature, it would not be possible for God to share the latter with His offspring while retaining the former exclusively to Himself. Either both would be shared, making the Son the same individual identity as the Father and thus no son at all, or else neither could be shared, since they are indistinguishable and inseparable, actually being one and the same thing within a truly simple being.
Of course, the idea that God is simple is more philosophical than biblical, and many have challenged it on rigorous philosophical and exegetical grounds. But even if the doctrine of divine simplicity is not correct, even the fact that it may exist as a possibility serves to illustrate the point that we aren’t justified in simply assuming that God has a nature or species or essence distinguishable from His person. While it certainly true that it is universal among creatures for a father to pass on a species to a son in procreation, there is no reason that the same must be so for God (just as we would say in respect to the corporeal, sexual, or temporal nature of creaturely generation). We must recall that God created various beings, men, and animals, to reproduce each after their kind, just as He made them male and female with the ability for sexual reproduction, and gave them bodies. All of these things are part of the way they are designed, and the reproductive aspects exist for the continuation of these species upon the death of their parents. But an immortal, eternal, uncreated, and incorporeal God has no need for any of these features, including a communicable species.
When we further consider how differences between God and His creatures might impact the possibility of God having and passing on a communicable species to a literal offspring of some sort, we find the idea appears quite improbable. Firstly, we must consider that if it is indeed necessary and proper to the Father to generate the Son, as Monarchian Trinitarians claim, then it would seem that this necessity to generate another would be quite essential to Him. If that is so, then when His essence is communicated to another, it would appear that it must include this necessity of begetting another; which should lead to the offspring likewise eternally generating another offspring, and so on and so on infinitely. This infinite chain of gods will result in far more than a trinity of persons. If the Father’s generation of the Son were merely a voluntary act of will (as such generation is with creatures), then we might well be able to imagine the process stopping, due to God’s willing it to, at only one Father and one Son. But since Monarchian Trinitarians deny that the Father’s generation is an act of will, and instead make it a necessary and eternal function of the Father, this solution will not be available to them.
Secondly, we must consider that there is no obvious sense in which all the “essential” attributes of God are communicable. Many a theologian and philosopher have argued convincingly that it is an impossibility for there to be more than one infinite entity, for example, making the property of infinitude incommunicable; but of course, the Father generating a Son that is equally infinite, makes two infinites. ‘Classical theism’ argues this is impossible according to the very nature of infinitude, but Monarchian Trinitarianism proposes exactly this anyway. Because humans were designed by God to have a communicable kind or species to pass on to their offspring, of course all essential human properties are communicable; but we have no reason to think this is the case with God, Who has properties (like infinitude) that are far different than those found in creatures.
In sum, then, we find that the “plain and simple” christology of Monarchian Trinitarianism is anything but that. There is nothing straightforward about insisting that God must father a son in exactly the same fashion as a man begets a son, and then turning around and denying almost all similarities between the two. There’s nothing ‘so simple a child can understand’ about the one God having a communicable species like men do that He passes on in the generation of the Son, while this generation is totally unlike that of men in so many other respects, that really, if anyone extends the analogy the slightest bit further, such as proposing that God is corporeal or that the generation is temporal, they are cast straight into the realm of heresy. A child would not make such distinctions so as to arbitrarily pick out co-essentiality from sonship while leaving aside all the other connotations sonship carries. We can’t say it’s a simple matter of looking at what a father-son relationship is in creation, and then go and say it isn’t really anything like the father-son relationships we see in creation. Yet this is precisely what many Monarchian Trinitarians appear to do.
We would do well to recall that the inspired writings of the Old Testament, which should provide our background for understanding the New, never present divine sonship as ontological; men like Solomon and David were called ‘the son of God’ as well as the future Messiah (who was always foretold to be a man, a descendant of David). Yet obviously David and Solomon are not eternally begotten by God, even though they’re each able to be plainly spoken of as God’s son. That’s because ‘son of God’ was a kingly title, indicating the position of Israel’s king as God’s ‘right hand’ ruler, himself subject to the God of Israel, but after Him exalted by God over the whole earth. Thus the coming Messiah was foretold to be, like David, God’s Son; not as being eternally begotten by God as some sort of identical clone of God, but as God’s specially appointed human king over the earth. When we look at what the scriptures mean by the title ‘son of God’, it becomes clear that there is in fact no implications of a shared nature or ontological generation from God at all.