We Believe in the Only Begotten Son, Part II

1. The Son of God Revealed in the Holy Scriptures

87. At  the  Theophany  on  the  Jordan  River,  the  Father  himself  calls  Jesus  Christ his own Son: “You are my Son, the Beloved; with you I am well pleased” (Mk 1:11). In the Old Testament, the title son of God is given to the people of God: “Thus says the Lord, Israel is my first-born son” (Ex  4:22).  The  title  is  also  given  to  the  Messiah,  the  Anointed  One:  “The Lord said to me, ‘You are my son, today I have begotten you. Ask of me, and I will make the nations your heritage, and the ends of the earth your possession’ ” (Ps 2:7-8).
88. The New Testament gives new meaning to the title Son of God, empha-sizing the uniqueness of the relationship between Jesus Christ and God the Father. Christ himself makes a distinction between his own divine sonship  and  our  sonship  in  God:  “I  am  ascending  to  my  Father  and  your Father” (Jn 20:17; see Jn 1:18). The Father witnesses to the exclusiveness  of  Christ’s  sonship  in  the  revelation  at  the  Transfiguration:  “This is my Son, my Chosen; listen to him!” (Lk 9:35). The evangelist John professes the divinity of God’s Son: “The Son of God has come and has given us understanding so that we may know him who is true ... he is the true God and eternal life” (1 Jn 5:20)

2. The Only-Begotten Son

89. The  Son  of  God,  the  Second  Person  of  the  Most  Holy  Trinity,  equal  in dignity and essence with the Father, is “the splendour of the Father, the unchangeable and immutable image of his essence and his nature, the fountain of wisdom and grace.”(Floral Triodion, Monday of the Holy Spirit, Vespers, Second Kneeling Prayer). Therefore, the Church professes Christ to be the only-begotten Son, in the bosom of the Father (see Jn1:18). Begotten of the Father, the Son is uncreated; he is the only One from the Father, an unrepeatable Person of God, the Word of God.For  God  sent  forth  the  Word  ...  just  as  the  root  puts  forth  the  tree, and the fountain [i.e., spring] the river, and the sun the ray ... should not hesitate, indeed, to call the tree the son or offspring of  the  root,  and  the  river  that  of  the  fountain  [i.e.,  spring],  and  the ray that of the sun; because every original source is a parent, and everything which issues from the origin is an offspring. Much more is this true of the Word of God, who has actually received as his own peculiar designation the name of Son.(Tertullian, Against Praxeas, 8)
90. The  Church  professes  the  Son  to  be  one  in  being  [essence]  with  the  Father,  emphasizing  the  equality  of  Persons  within  the  Most  Holy  Trinity, and the unity of God’s nature, thus confirming the divinity of the  Son  of  God.  The  Son  did  not  come  to  be  in  time,  as  did  crea-tures. Therefore, he is not lower than the Father. He co-exists with him eternally and is equal to him in all things, other than begetting, which belongs to the Father.(See John of Damascus, Exact Exposition of the Orthodox Faith, I, 7)

From Christ Our Pascha, 

the Catechism of the Ukrainian Catholic Church