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
1 The Word is God from God; for 'the Word was God,' and again, 'Of whom are the Fathers, and of whom Christ, who is God over all, blessed for ever. Amen.' And since Christ is God from God, and God's Word, Wisdom, Son, and Power, therefore but One God is declared in the divine Scriptures. For the Word, being Son of the One God, is referred to Him of whom also He is; so that Father and Son are two, yet the Monad of the Godhead is indivisible and inseparable. And thus too we preserve One Beginning of Godhead and not two Beginnings, whence there is strictly a Monarchy. And of this very Beginning the Word is by nature Son, not as if another beginning, subsisting by Himself, nor having come into being externally to that Beginning, lest from that diversity a Dyarchy and Polyarchy should ensue; but of the one Beginning He is own Son, own Wisdom, own Word, existing from It. For, according to John, 'in' that 'Beginning was the Word, and the Word was with God,' for the Beginning was God; and since He is from It, therefore also 'the Word was God.' And as there is one Beginning and therefore one God, so one is that Essence and Subsistence which indeed and truly and really is, and which said 'I am that I am,' and not two, that there be not two Beginnings; and from the One, a Son in nature and truth, is Its own Word, Its Wisdom, Its Power, and inseparable from It. And as there is not another essence, lest there be two Beginnings, so the Word which is from that One Essence has no dissolution, nor is a sound significative, but is an essential Word and essential Wisdom, which is the true Son. For were He not essential, God will be speaking into the air, and having a body, in nothing differently from men; but since He is not man, neither is His Word according to the infirmity of man. For as the Beginning is one Essence, so Its Word is one, essential, and subsisting, and Its Wisdom. For as He is God from God, and Wisdom from the Wise, and Word from the Rational, and Son from Father, so is He from Subsistence Subsistent, and from Essence Essential and Substantive, and Being from Being.
2. Since were He not essential Wisdom and substantive Word, and Son existing, but simply Wisdom and Word and Son in the Father, then the Father Himself would have a nature compounded of Wisdom and Word. But if so, the forementioned absurdities would follow; and He will be His own Father, and the Son begetting and begotten by Himself; or Word, Wisdom, Son, is a name only, and He does not subsist who owns, or rather who is, these titles. If then He does not subsist, the names are idle and empty, unless we say that God is Very Wisdom and Very Word. But if so, He is His own Father and Son; Father, when Wise, Son, when Wisdom; but these things are not in God as a certain quality; away with the dishonourable thought; for it will issue in this, that God is compounded of essence and quality. For whereas all quality is in essence, it will clearly follow that the Divine Monad, indivisible as it is, must be compound, being severed into essence and accident. We must ask then these headstrong men; The Son was proclaimed as God's Wisdom and Word; how then is He such? If as a quality, the absurdity has been shown; but if God is that Very Wisdom, then it is the absurdity of Sabellius; therefore He is so, as an Offspring in a proper sense from the Father Himself, according to the illustration of light. For as there is light from fire, so from God is there a Word, and Wisdom from the Wise, and from the Father a Son. For in this way the Monad remains undivided and entire, and Its Son, Word not unessential, nor not subsisting, but essential truly. For were it not so, all that is said would be said notionally and verbally. But if we must avoid that absurdity, then is a true Word essential. For as there is a Father truly, so Wisdom truly. In this respect then they are two; not because, as Sabellius said, Father and Son are the same, but because the Father is Father and the Son Son, and they are one, because He is Son of the Essence of the Father by nature, existing as His own Word. This the Lord said, viz. 'I and the Father are One;' for neither is the Word separated from the Father, nor was or is the Father ever Wordless; on this account He says, 'I in the Father and the Father in Me. '
3. And again, Christ is the Word of God. Did He then subsist by Himself, and subsisting, has He become joined to the Father, or did God make Him or call Him His Word? If the former, I mean if He subsisted by Himself and is God, then there are two Beginnings; and moreover, as is plain, He is not the Father's own, as being not of the Father, but of Himself. But if on the contrary He be made externally, then is He a creature. It remains then to say that He is from God Himself; but if so, that which is from another is one thing, and that from which it is, is a second; according to this then there are two. But if they be not two, but the names belong to the same, cause and effect will be the same, and begotten and begetting, which has been shown absurd in the instance of Sabellius. But if He be from Him, yet not another, He will be both begetting and not begetting; begetting because He produces from Himself, and not begetting, because it is nothing other than Himself. But if so, the same is called Father and Son notionally. But if it be unseemly so to say, Father and Son must be two; and they are one, because the Son is not from without, but begotten of God. But if any one shrinks from saying 'Offspring,' and only says that the Word exists with God, let such a one fear lest, shrinking from what is said in Scripture, he fall into absurdity, making God a being of double nature. For not granting that the Word is from the Monad, but simply as if He were joined to the Father, he introduces a twofold essence, and neither of them Father of the other. And the same of Power. And we may see this more clearly, if we consider it with reference to the Father; for there is One Father, and not two, but from that One the Son. As then there are not two Fathers, but One, so not two Beginnings, but One, and from that One the Son essential.
Source: Four Discourses Against the Arians (New Advent)