Well, Catholics know it because the CC has said it.
We profess it every time we say the Nicene Creed.
"The (early Church Fathers) recognized that the Bible depicts the Son as having his identity as the Son before his incarnation. In 1 John 4:9 we read, that “the love of God was made manifest among us [in] that God sent his only Son into the world, so that we might live through him.” Thus, the second person of the Trinity was already the Son when he was sent into the world.
ECFs:
Ignatius of Antioch
“Jesus Christ . . . was with the Father before the beginning of time, and in the end was revealed” (Letter to the Magnesians 6 [A.D. 110]).
Justin Martyr
“Jesus Christ is the only proper Son who has been begotten by God, being his Word and first-begotten, and power; and, becoming man according to his will, he taught us these things for the conversion and restoration of the human race” (First Apology 23 [A.D. 151]).
“God begot before all creatures a beginning, who was a certain rational power from himself and whom the Holy Spirit calls . . . sometimes the Son
. . . sometimes Lord and Word. . . . We see things happen similarly among ourselves, for whenever we utter some word, we beget a word, yet not by any cutting off, which would diminish the word in us when we utter it. We see a similar occurrence when one fire enkindles another. It is not diminished through the enkindling of the other, but remains as it was” (Dialogue with Trypho the Jew 61 [A.D. 155]).
Irenaeus
“[The Gnostics] transfer the generation of the uttered word of men to the eternal Word of God, attributing to him a beginning of utterance and a coming into being . . . . In what manner, then, would the Word of God—indeed, the great God himself, since he is the Word—differ from the word of men?” (Against Heresies 2:13:8 [A.D. 189]).
Tertullian
“The Father makes him equal to himself, and the Son, by proceeding from him, was made the first-begotten, since he was begotten before all things, and the only-begotten, because he alone was begotten of God, in a manner peculiar to himself, from the womb of his own heart, to which even the Father himself gives witness: ‘My heart has poured forth my finest Word’ [Ps. 45:1–2]” (Against Praxeas 7:1 [A.D. 216]).
Hippolytus
“Therefore, this sole and universal God, by reflecting, first brought forth the Word—not a word as in speech, but as a mental word, the reason for everything. . . . The Word was the cause of those things which came into existence, carrying out in himself the will of him by whom he was begotten. . . . Only [God’s] Word is from himself and is therefore also God, becoming the substance of God” (Refutation of All Heresies 10:33 [A.D. 228]).
Origen
“So also Wisdom, since he proceeds from God, is generated from the very substance of God” (Commentary on Hebrews [A.D. 237]).
Gregory the Wonderworker
“There is one God, the Father of the living Word, who is his subsistent wisdom and power and eternal image: perfect begetter of the perfect begotten, Father of the only-begotten Son. There is one Lord, only of the only, God of God, image and likeness of deity, efficient Word, wisdom comprehensive of the constitution of all things, and power formative of the whole creation, true Son of true Father” (Declaration of Faith [A.D. 265]).
Lactantius
“When we speak of God the Father and God the Son, we do not speak of them as different, nor do we separate them, because the Father cannot exist without the Son, nor can the Son be separated from the Father, since the name of ‘Father’ cannot be given without the Son, nor can the Son be begotten without the Father. . . . [T]hey both have one mind, one spirit, one substance; but the former [the Father] is as it were an overflowing fountain, the latter [the Son] as a stream flowing forth from it. The former as the sun, the latter as it were a ray [of light] extended from the sun” (Divine Institutes 4:28–29 [A.D. 307]).
Council of Nicaea I
[SIGN]“We believe . . . in our one Lord Jesus Christ the Son of God, the only-begotten born of the Father, that is, of the substance of the Father, God of God, light of light, true God of true God, begotten, not made . . .” [/SIGN](The Creed of Nicaea [A.D. 325]).
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