This Sunday we celebrate the Solemnity of the Most Holy Trinity. It is one of the few feasts that celebrates a doctrine rather than an event. The liturgical color is white.
Pope Gregory IX instituted Trinity Sunday in 828 AD. This day is dedicated to the Christian belief in the Trinity – Father, Son and Holy Spirit – and falls on the Sunday following Pentecost each year. The celebration of Trinity Sunday was made universal by Pope John XXIII.
The origins of the celebration of Trinity Sunday go all the way back to the Arian heresy of the fourth century. Arius believed that Christ was a created being and in denying the divinity of Christ, he denied that there are three persons in one God. Arius’ chief opponent, Athanasius, upheld the orthodox doctrine that there are three persons in one God and the orthodox view prevailed at the Council of Nicea. For many centuries, the Athanasian Creed was recited at Mass on Trinity Sunday.
Commentary by Hilary of Poitiers: The Trinity XII, 55-56: PL 10, 468-472
Hilary (315-367); Doctor of the Church – was elected bishop of Poitiers in 353. Because of his struggles with the Arians and his treatise on the Trinity, for which he was exiled, he has been called “the Athanasius of the West.”
According to the apostle, Lord, your Holy Spirit fully understands and penetrates your inmost depths; he also intercedes on my behalf, saying to you things for which I cannot find the words.
Your Holy Spirit proceeds through your Son from you; though I may fail to grasp the full meaning of that statement, I give it nonetheless the firm assent of my mind and heart.
I may indeed show dullness and stupidity in my understanding of these spiritual matters; it is as your only Son has said: “Do not be surprised if I have said to you: You must be born again. Just as the wind blows where it pleases and you hear the sound of it without knowing where it is coming from or going to, so will it be with everyone who is born again of water and the Holy Spirit.”
By my regeneration I have received the faith, but I am still ignorant; and yet I have a firm hold on something which I do not understand. I am born again, capable of rebirth but without conscious perception of it.
The Spirit abides by no rules; he speaks when he pleases, what he pleases, and where he pleases. We are conscious of his presence when he comes, but the reasons for his approach or his departure remain hidden from us.
John tells us that all things came into being through the Son who is God the Word abiding with you, Father, from the beginning. Paul in his turn enumerates the things created in the Son, both visible and invisible, in heaven and on earth. And while he is specific about all that was created in and through Christ, of the Holy Spirit he considers it enough simply to say that he is your Spirit.
Therefore I concur with those chosen men in thinking that just as it is not expedient for me to venture beyond my mental limitation and predicate anything of your only begotten Son save that, as those witnesses have assured us, he was born of you, so it is not fitting for me to go beyond the power of human thought and the teaching of those same witnesses by declaring anything regarding the Holy Spirit other than that he is your Spirit.
Rather than waste time in a fruitless war of words, I would prefer to spend it in the firm profession of an unhesitating faith. I beg you therefore, Father, to preserve in me that pure and reverent faith and to grant that to my last breath I may testify to my conviction.
May I always hold fast to what I publicly professed in the creed when I was baptized in the name of the Father and of the Son and of the Holy Spirit. May I worship you, the Father of us all, and your Son together with you and may I be counted worthy to receive your Holy Spirit who through your only Son proceeds from you.
Readings for the Solemnity can be found here