This Sunday is The Solemnity of the Most Holy Trinity on the liturgical calendar.
For whatever reason many Christians struggle with the doctrine of the Holy Trinity. There is even the story of Saint Augustine walking on the beach contemplating the mystery of the Trinity. Then he saw a boy in front of him who had dug a hole in the sand and was going out to the sea again and again and bringing some water to pour into the hole. St. Augustine asked him, “What are you doing?” “I’m going to pour the entire ocean into this hole.” “That is impossible, the whole ocean will not fit in the hole you have made” said St. Augustine. The boy replied, “And you cannot fit the Trinity in your tiny little brain.” The story concludes by saying that the boy vanished as St. Augustine had been talking to an angel.
It wasn’t a difficult doctrine for me to accept once I prayed for the faith to believe it. What God did for me was to have me approach the doctrine through love. Not that the love of God is something I could grasp either, but the reality of love is the only way that I could make sense out of the Holy Trinity. That is to say, that God is Triune because God is Love (Cf. 1 Jn. 4:8). God being love, out of that Love came His only begotten Son Christ Jesus, and out of their love flowed the Holy Spirit.
Love cannot be contained unto itself – the first principle of love is that love must be given away as freely and liberally as it was first received. Therefore, God being the author and source of Love is also fountain of that Divine Love, which never stops flowing. Jesus, being everything like His Father (Love) equally gives Himself away, and out of that mutual outpouring of Love between the Father and the Son came the Holy Spirit; Love from Love, Light from Light, True God from True God. But the Divine Principle of Love didn’t end there; for, out of their Love came creation, and while the angels were given to share love in service of God, we humans were given to share God’s love in both service and procreation. Indeed, love is the source of all life and the source of all life is the Holy Trinity (Father, Son, Holy Spirit).
Official promulgated by the Church Fathers at the First Council of Nicaea in 325, today that same Church expresses Her belief in the Triune God in this order of words found in the Catechism of the Catholic Church:

    253 The Trinity is One. We do not confess three Gods, but one God in three persons, the “consubstantial Trinity” (Cf. Council of Constantinople II (553): DS 421). The divine persons do not share the one divinity among themselves but each of them is God whole and entire: “The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God” (Cf. Council of Toledo XI (675): DS 530:26). In the words of the Fourth Lateran Council (1215), “Each of the persons is that supreme reality, viz., the divine substance, essence or nature” (Cf. Lateran Council IV (1215): DS 804).
    254 The divine persons are really distinct from one another. “God is one but not solitary” (Cf. Fides Damasi: DS 71). “Father”, “Son”, “Holy Spirit” are not simply names designating modalities of the divine being, for they are really distinct from one another: “He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son” (Cf. Council of Toledo XI (675): DS 530:25). They are distinct from one another in their relations of origin: “It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds” (Cf. Lateran Council IV (1215): DS 804). The divine Unity is Triune.
    255 The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: “In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance” (Cf. Council of Toledo XI (675): DS 528). Indeed “everything (in them) is one where there is no opposition of relationship” (Cf. Council of Florence (1442): DS 1330). “Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son” (Council of Florence (1442): DS 1331).

      256 St. Gregory of Nazianzus, also called “the Theologian”, entrusts this summary of Trinitarian faith to the catechumens of Constantinople:
      Above all guard for me this great deposit of faith for which I live and fight, which I want to take with me as a companion, and which makes me bear all evils and despise all pleasures: I mean the profession of faith in the Father and the Son and the Holy Spirit. I entrust it to you today. By it I am soon going to plunge you into water and raise you up from it. I give it to you as the companion and patron of your whole life. I give you but one divinity and power, existing one in three, and containing the three in a distinct way. Divinity without disparity of substance or nature, without superior degree that raises up or inferior degree that casts down. . . the infinite co-naturality of three infinites. Each person considered in himself is entirely God. . . the three considered together. . . I have not even begun to think of unity when the Trinity bathes me in its splendor. I have not even begun to think of the Trinity when unity grasps me. . . (St. Gregory of Nazianzus, Oratio 40,41: PG 36,417)

Have you ever prayed the Holy Trinity Rosary Invocation?
Below are two of my poems on the Holy Trinity from Chapter Twelve of my book Cooperating with God: The Bridegroom’s Prayer

Silent Prayer
So I contemplate,
There are things that I do see and feel,
and there are things that I neither see nor feel,
the dark cloud,
the purifying light,
the unnecessary distractions of the enemy.
So I pray, ‘God our Father draw me nearer to thee’.
Next, there is that energy that I feel, but do not see.
So I pray, ‘Christ Jesus transubstantiate my flesh with thees.
Then there is the inner work of grace
that my senses cannot perceive,
but through the evidence of faith – I do believe!
So I pray, ‘Holy Spirit conform my will to thees.
The Presence of the Triune God
Oh, look, there is a stream,
but where does it begin?
And there is a river,
but where does it end?
There is the sun,
but when will it dim?
Oh, here is the Triune God,
who is without beginning or end,
whose love for me will never dim,
and whose divinity I desire to fully dwell in.