Today’s Catechism sections discuss Jesus’ title of  “The Only Son of God”. Supporting material comes from St. Augustine’s “Sermon to Catechumens on the Creed”.

III. The Only Son of God

441 In the Old Testament, "son of God" is a title given to the angels, the Chosen People, the children of Israel, and their kings.44 It signifies an adoptive sonship that establishes a relationship of particular intimacy between God and his creature. When the promised Messiah-King is called "son of God", it does not necessarily imply that he was more than human, according to the literal meaning of these texts. Those who called Jesus "son of God", as the Messiah of Israel, perhaps meant nothing more than this.45

442 Such is not the case for Simon Peter when he confesses Jesus as "the Christ, the Son of the living God", for Jesus responds solemnly: "Flesh and blood has not revealed this to you, but my Father who is in heaven."46 Similarly Paul will write, regarding his conversion on the road to Damascus, "When he who had set me apart before I was born, and had called me through his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles..."47 "and in the synagogues immediately [Paul] proclaimed Jesus, saying, 'He is the Son of God.'"48 From the beginning this acknowledgment of Christ's divine sonship will be the centre of the apostolic faith, first professed by Peter as the Church's foundation.49

443 Peter could recognize the transcendent character of the Messiah's divine sonship because Jesus had clearly allowed it to be so understood. To his accusers' question before the Sanhedrin, "Are you the Son of God, then?" Jesus answered, "You say that I am."50 Well before this, Jesus referred to himself as "the Son" who knows the Father, as distinct from the "servants" God had earlier sent to his people; he is superior even to the angels.51 He distinguished his sonship from that of his disciples by never saying "our Father", except to command them: "You, then, pray like this: 'Our Father'", and he emphasized this distinction, saying "my Father and your Father".52

444 The Gospels report that at two solemn moments, the Baptism and the Transfiguration of Christ, the voice of the Father designates Jesus his "beloved Son".53 Jesus calls himself the "only Son of God", and by this title affirms his eternal pre-existence.54 He asks for faith in "the name of the only Son of God".55 In the centurion's exclamation before the crucified Christ, "Truly this man was the Son of God",56 that Christian confession is already heard. Only in the Paschal mystery can the believer give the title "Son of God" its full meaning.

445 After his Resurrection, Jesus' divine sonship becomes manifest in the power of his glorified humanity. He was "designated Son of God in power according to the Spirit of holiness by his Resurrection from the dead".57 The apostles can confess: "We have beheld his glory, glory as of the only Son from the Father, full of grace and truth."58

St. Augustine discusses the “Father Almighty’s Only Son, our Lord” in his “Sermon to Catechumens on the Creed”:

3. For this reason we believe also in His Son, that is to say, God the Father Almighty's, His Only Son, our Lord. When you hear of the Only Son of God, acknowledge Him God. For it could not be that God's Only Son should not be God. What He is, the same did He beget, though He is not that Person Whom He begot. If He be truly Son, He is that which the Father is; if He be not that which the Father is, He is not truly Son. Observe mortal and earthly creatures: what each is, that it engenders. Man besets not an ox, sheep besets not dog, nor dog sheep. Whatever it be that begets, that which it is, it begets. Hold ye therefore boldly, firmly, faithfully, that the Begotten of God the Father is what Himself is, Almighty. These mortal creatures engender by corruption. Does God so beget? He that is begotten mortal generates that which himself is; the Immortal generates what He is: corruptible begets corruptible, Incorruptible begets Incorruptible: the corruptible begets corruptibly, Incorruptible, Incorruptibly: yea, so begets what Itself is, that One begets One, and therefore Only. You know, that when I pronounced to you the Creed, so I said, and so you are bounden to believe; that we believe in God the Father Almighty, and in Jesus Christ His Only Son. Here too, when you believe that He is the Only, believe Him Almighty: for it is not to be thought that God the Father does what He will, and God the Son does not what He will. One Will of Father and Son, because one Nature. For it is impossible for the will of the Son to be any whit parted from the Father's will. God and God; both one God: Almighty and Almighty; both One Almighty.

Footnotes

44 Cf. Dt 14:1; (LXX) 32:8; ⇒ Job 1:6; Ex 4:22; ⇒ Hos 2:1; ⇒ 11:1; ⇒ Jer 3:19; ⇒ Sir 36:11; Wis 18:13; 2 Sam 7:14; ⇒ Ps 82:6.
45 Cf. I Chr 17:13; ⇒ Ps 2:7; ⇒ Mt 27:54; ⇒ Lk 23:47.
46 ⇒ Mt 16:16-17.
47 ⇒ Gal 1:15-16.
48 ⇒ Acts 9:20.
49 Cf. I Th 1:10; ⇒ Jn 20:31; ⇒ Mt 16:18.
50 ⇒ Lk 22:70; cf. ⇒ Mt 26:64; ⇒ Mk 14:61-62.
51 Cf. ⇒ Mt 11:27; ⇒ 21:34-38; ⇒ 24:36.
52 ⇒ Mt 5:48; ⇒ 6:8-9; ⇒ 7:21; ⇒ Lk 11:13; ⇒ Jn 20:17.
53 Cf. ⇒ Mt 3:17; cf. ⇒ 17:5.
54 ⇒ Jn 3:16; cf. ⇒ 10:36.
55 ⇒ Jn 3:18.
56 ⇒ Mk 15:39.
57 ⇒ Rom 1:3; cf. ⇒ Acts 13:33.
58 ⇒ Jn 1:14.