The Father without Beginning

79. The Church calls “Father” him who has no father, and was never anyone’s son (See Athanasius the Great, First Letter to Serapion, 16) The evangelist John says that the Word was “in the beginning”  ( Jn  1:1)  (in  Slavonic, в начали, v  nachali,  “as  the  foundational  principle”).  The Holy Fathers understood this to affirm that “the beginning” is the Person of the Father (Gregory the Theologian, Oration 42, 15). He who is the personal Beginning of all is himself without a beginning that would cause him to be. In other words, the Father  is  the  Beginning  without  beginning.  This  antinomy  expresses  the freedom of the divine Person of the Father, who is not caused by anything or anyone. “The Father is ... said to be both without origin, and origin  himself—origin  in  that  he  is  the  cause  and  spring  and  eternal.
80. In her experience, the Church always contemplates the Father in relation to the Son and the Holy Spirit, and this Trinity (Triad) as a communion of Persons.I believe in one God, the Father glorified in the Trinity—unbegotten, beginningless and endless; in the Son—begotten, yet co-beginningless  and  co-endless  with  the  Father;  and  in  the  Holy  Spirit—who proceeds from the Father and is revealed in the Son, co-beginningless and equal to the Father and the Son. I believe in the Trinity, one in essence, yet multiple in Persons; Trinity accord-ing to names, yet one God (Ilarion, Metropolitan of Kyiv, Confession of Faith).
81. The Father is the Father because he is the Father of the Son and the Source  of  the  procession  of  the  Holy  Spirit.  The  Son  and  the  Holy  Spirit are “from the Father, although not after the Father.”(See Gregory the Theologian, Oration 29, The Third Theological Oration, 3). The generation of the Word and the procession of the Spirit are not to be taken according  to  categories  of  time:  “For  the  Father  eternally  begets  the  Son, co-eternal and co-reigning; and the Holy Spirit is in the Father, glorified with the Son; one power, one nature, and one divinity.”(Floral Triodion, Pentecost, Vespers, Final Sticheron at Psalm 140). God the Father is the Beginning of the indivisible and most intimate communion (in Greek, koinonia) in love of the divine Persons.
82. The personal distinctiveness of the Father from the Son and the Holy Spirit lies in the fact that the Father is unbegotten. Himself unbegotten, he is the Beginning of the Person of the Son and of the Person of the Holy Spirit. The fact that God the Father is without beginning means that he is not conditioned by anyone or anything else.The Father is called thus because he begets the Son; and the Son is called thus because he is begotten by the Father; and the Holy Spirit is called thus because of his procession from the Father, yet being inseparable from him (Ilarion, Metropolitan of Kyiv, Confession of Faith).
83. In the Anaphora of the Liturgy of Saint Basil the Great, the Church calls  the  Father  incomprehensible  and  uncircumscribable. The incomprehensibility of the Father indicates that no stage of our coming to know God  is  ever  definitive.  After  all,  only  God  can  know  God  to  his  very  depths: “no one comprehends the thoughts of God except the Spirit of God” (1 Cor 2:11). Our knowledge of God will grow in faith, according to the words of the holy apostle Paul, until we see him “face to face” (1 Cor 13:12). For humankind, the incomprehensibility of God guarantees an endless growth in coming to know God, which is “eternal life.”
84. Just as the Father cannot be circumscribed, so also his incomprehensibility attests to the impossibility of reducing the Personhood of God to any forms of expression used by human beings, whether through image or word. God the Father is not portrayed on icons, because the Father did not become incarnate, nor take on the image of a human being. The Father is revealed by his only-begotten Son, who is the “image of the invisible God” (Col 1:15).

From Christ Our Pascha, 

the Catechism of the Ukrainian Catholic Church