We Believe in the Trinity, One in Essence and Undivided

From Christ Our Pascha, 

the Catechism of the Ukrainian Catholic Church

71 The Mystery of the Most Holy Trinity—the Father, the Son, and the Holy Spirit—infinitely surpasses all possibilities of human understanding. “His divinity [i.e., Godhead] is completely unfathomable, and his essence, which is above all, is not what the mind conceives it to be.”(St Gregory of Nyssa, Oration on the Divinity of the Son and the Holy Spirit). According to the words of the apostle Paul, the depths of this mystery can be plumbed only by the Holy Spirit: “So also no one comprehends what  is  truly  God’s  except  the  Spirit  of  God”  (1  Cor  2:11).  However,  that same Spirit, the Spirit of truth, descending upon people, reveals the inner life of God. The Holy Spirit teaches people to express the truth, inasmuch  as  this  is  possible,  through  the  language  of  faith:  “Now  we  have received ... the Spirit that is from God, so that we may understand the  gifts  bestowed  upon  us  by  God.  And  we  speak  of  these  things  in  words not taught by human wisdom but taught by the Spirit, interpreting  spiritual  things  to  those  who  are  spiritual”  (1  Cor  2:12-13).  And  then, having thus taught, the Holy Spirit keeps vigil, that the faith, “the assurance of things hoped for,” might grow into the contemplation of “things not seen” (see Heb 11:1); that humankind may see the invisible as if it were visible, and see what is obscure face to face. “[Christians of the East] perceive that one draws close to this presence above all by let-ting oneself be taught an adoring silence, for at the culmination of the knowledge and experience of God is his absolute transcendence.” (St. John Paul II, Orientale Lumen)

72  The presence of the transcendent God is expressed through the symbolism of the liturgy and of the church building: by the curtain behind the Royal Doors of the iconostasis, by the Holy Gifts covered by veils, by the raised aer (in Slavonic, vozdukh, вождух) during the Creed, and by the cloud of incense. “There is the transcendent unity of God and the fruitfulness of God, and as we prepare to sing this truth we use the names ‘Trinity’ and ‘Unity’ for that which is fact beyond every name.”(Pseudo-Dionysius the Areopagite, The Divine Names) Even though we refer to God using these titles, we are fully aware that we do not thereby exhaust the mysteries of God. No sooner do I conceive of the One than I am illumined by the splendour of the Three; no sooner do I distinguish them [as Per-sons] than I am carried back to the One. When I think of any One of the Three [Persons] I think of him as the whole, and my eyes are filled, and the greater part of what I am thinking of escapes me. (Gregory the Theologian, The Oration on Holy Baptism).

73  In our liturgical and personal prayers, through the constant invocation and glorification of the name of the Father, and the Son, and the Holy Spirit,  we  simultaneously  express  our  faith  and  communion  with  the  Most Holy Trinity, which grants us “the grace of the Lord Jesus Christ, the love of God the Father, and the fellowship of the Holy Spirit.” (The Divine Liturgy of St. John Chrysostom, Blessing prior to the Anaphora).