5. The substantiality of the Word proved from Scripture. If the One Origin be substantial, Its Word is substantial. Unless the Word and Son be a second Origin, or a work, or an attribute (and so God be compounded), or at the same time Father, or involve a second nature in God, He is from the Father's Essence and distinct from Him. Illustration of John 10:30, drawn from Deuteronomy 4:4
4 But the Arians we must ask contrariwise: (for the Sabellianisers must be confuted from the notion of a Son, and the Arians from that of a Father:) let us say then— Is God wise and not word-less: or on the contrary, is He wisdom-less and word-less? If the latter, there is an absurdity at once; if the former, we must ask, how is He wise and not word-less? Does He possess the Word and the Wisdom from without, or from Himself? If from without, there must be one who first gave to Him, and before He received He was wisdom-less and word-less. But if from Himself, it is plain that the Word is not from nothing, nor once was not; for He was ever; since He of whom He is the Image, exists ever. But if they say that He is indeed wise and not word-less, but that He has in Himself His own wisdom and own word, and that, not Christ, but that by which He made Christ, we must answer that, if Christ in that word was brought to be, plainly so were all things; and it must be He of whom John says, 'All things were made by Him,' and the Psalmist, 'In Wisdom have You made them all. ' And Christ will be found to speak untruly, 'I in the Father,' there being another in the Father. And 'the Word became flesh ' is not true according to them. For if He in whom 'all things came to be,' Himself became flesh, but Christ is not in the Father, as Word 'by whom all things came to be,' then Christ has not become flesh, but perhaps Christ was named Word. But if so, first, there will be another besides the name, next, all things were not by Him brought to be, but in that other, in whom Christ also was made. But if they say that Wisdom is in the Father as a quality or that He is Very Wisdom, the absurdities will follow already mentioned. For He will be compound, and will prove His own Son and Father. Moreover, we must confute and silence them on the ground, that the Word which is in God cannot be a creature nor out of nothing; but if once a Word be in God, then He must be Christ who says, 'I am in the Father and the Father in Me,' who also is therefore the Only-begotten, since no other was begotten from Him. This is One Son, who is Word, Wisdom, Power; for God is not compounded of these, but is generative of them. For as He frames the creatures by the Word, so according to the nature of His own Essence has He the Word as an Offspring, through whom He frames and creates and dispenses all things. For by the Word and the Wisdom all things have come to be, and all things together remain according to His ordinance. And the same concerning the word 'Son;' if God be without Son, then is He without Work; for the Son is His Offspring through whom He works; but if not, the same questions and the same absurdities will follow their audacity.
5. From Deuteronomy; 'But ye that did attach yourselves unto the Lord your God are alive every one of you this day.' From this we may see the difference, and know that the Son of God is not a creature. For the Son says, 'I and the Father are One,' and, 'I in the Father, and the Father in Me;' but things originate, when they make advance, are attached unto the Lord. The Word then is in the Father as being His own; but things originate, being external, are attached, as being by nature foreign, and attached by free choice. For a son which is by nature, is one with him who begot him; but he who is from without, and is made a son, will be attached to the family. Therefore he immediately adds, 'What nation is there so great who has God drawing near unto them?' and elsewhere, 'I a God drawing near;' for to things originate He draws near, as being strange to Him, but to the Son, as being His own, He does not draw near, but He is in Him. And the Son is not attached to the Father, but co-exists with Him; whence also Moses says again in the same Deuteronomy, 'You shall obey His voice, and apply yourselves unto Him;' but what is applied, is applied from without.
Source: Four Discourses Against the Arians (New Advent)