Discussions of “self-existence” have historically been important to the debates surrounding trinitarian doctrine. That “self-existence” is an attribute of God, all parties agree. However, sharp disagreements have occurred over how this attribute of “self-existence” related to things like eternal generation.
Some have argued that since it is proper to the divine nature that the subject be self-existent, therefore, eternal generation cannot be true, since this teaches that the Son has His origin from the Father (such as many modern Protestants). Other have tried to modify the doctrine of eternal generation to attempt to say that the Son has His person from the Father in eternal generation, while rejecting the historical orthodox view that the Son also has His essence communicated to Him from the Father in eternal generation (such as Calvin, and others in the Reformed tradition who followed him). Others have ventured to deny the Son the attribute of self-existence since He is derived from the Father, having both His person and nature from the Father, in eternal generation (such as Samuel Clarke).
All of these explanations fall short because they all make the same error of not distinguishing between two distinct ideas in respect to self-existence: self-caused existence, and self-sustained existence. This distinction is both necessary logically, and proved by scripture. The distinction between these ideas should be a fairly straightforward one; there is a difference between being uncaused and being self-sustained in one’s existence.
We are told in scripture that not only did God create all things through His Son, but that He also upholds and sustains the existence of all things through His Son. For example, Colossians 1:17, speaking of Christ says “He is before all things, and in Him all things hold together.” (NAS). Similarly Hebrews 1:3 says “who being the brightness of His glory and the express image of His person, and upholding all things by the word of His power, when He had by Himself purged our sins, sat down at the right hand of the Majesty on high,” (NKJV). We see then that the existence of all creation is upheld by God through His Son; the implication being that the created universe is not self-existent. We would drop out of existence were it not for God’s continual and perpetual upholding of our existence by His own power through His Son.
The Son, however, is contrasted with creation in this way by scripture. Unlike we who must have our lives continually upheld and sustained by God, the Son is said to have “life in Himself”; self-sustained existence. Yet in the same passage of scripture that we are explicitly told that the Son has “life in Himself” we are also told that He does not have this quality from nothing, or without cause or origin, but from His Father:
“For as the Father has life in Himself, so He has granted the Son to have life in Himself,” (John 5:26 NKJV)
The Son then is taught by scripture to have self-sustained existence, life in Himself, just as the Father does. He has this divine attribute from the Father, however, showing that He does not have “self-existence” in terms of having an uncaused or self-caused existence, but has His existence, and even the quality of self-sustained existence, from the Father. Incidentally, this also proves that the essence of the Father, that is, His divine nature, was given to the Son in eternal generation, as He has this divine attribute, not merely His person, from the Father. Thus a communication of essence in eternal generation is proven in this important passage.
But the main thing to note here is that when scripture speaks of the Son’s self-existence, that revelation is given to us in such a way as to make clear that what is intended is not uncaused or self-caused existence, but self-sustained existence. Thus, as Christ has life and existence in Himself, He is able to give life to us men, according to the will of the Father.
The term “self-existence”, then, without further clarification and qualification, is unhelpful to these discussions. By not properly distinguishing between the distinct ideas of self-caused or uncaused existence, and self-sustained existence, the issue is over-simplified and the ideas are confounded in such a way that error inevitably ensues.
On a related note, self-caused or uncaused existence is not even a divine attribute; the Father’s attribute of having uncaused existence is not a matter of what He is, and thus, an aspect of His divine nature, but a matter of how He is what He is- He is what He is without cause or source. The Son is the same thing, meaning, He has the same divine nature, yet the “how” is different for the Son; He is what He is from the Father, while the Father is what He is from none. The point being, uncaused of self-caused existence is not a property of the divine nature, but a relational property belonging to the Father alone.
This stands in stark contrast to self-sustained existence, which does deal with ‘what’ the persons are, and is a positive attribute. As such it is proper the divine nature, and thus shared by all three persons, the Father having that quality of none, and the Son and Holy Spirit possessing it of the Father by eternal generation and procession, respectively.