"Perhaps the deepest, the most profound of all mysteries is the mystery of the Trinity. The Church teaches us that although there is only one God, yet, somehow, there are three Persons in God. The Father is God, the Son is God, the Holy Spirit is God, yet we do not speak of three Gods, but only one God. They have the same nature, substance, and being.

We came to know this immense mystery because Christ revealed it to us. Just before ascending He told them: "Go teach all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit" (Matthew 28:19). We know that these Three are not just different ways of looking at one person. For at the Last Supper, Jesus told us: "I came forth from the Father." So He is different from the Father. But He also promised: "If I go, I will send Him [the Paraclete] to you. . . . He will guide you to all truth" (John 16:28, 7, 13). So the Holy Spirit is also different.

Even though the Three Persons are One God, yet they are distinct: for the Father has no origin, He came from no one. But the Son is begotten, He comes from the Father alone. The Holy Spirit comes or proceeds from both the Father and the Son. These different relations of origin tell us there are three distinct Persons, who have one and the same divine nature.

Even though everything the Three Persons do outside the Divine nature is done by all Three, yet it is suitable that we attribute some works specially to one or the other Person. So we speak of the Father especially as the power of creation, of the Son as the wisdom of the Father, of the Holy Spirit as goodness and sanctification.

The two doctrines of the Trinity and the Incarnation are the foundation of Christian life and worship. By becoming man, God the Son offered us a share in the inner life of the Trinity. By grace, we are brought into the perfect communion of life and love which is God, Father, Son and Holy Spirit. This sharing in the life of the Trinity is meant to culminate in heaven, where we will see the three Persons face to face, united to them in unspeakable love."

Rev. William G. Most

The Solemnity of the Most Holy Trinity, has as object the greatest, the most profound and incomprehensible truth of our holy faith. The feast was introduced into the Roman Church by Pope John XXII in 1334.

The dogma of faith which forms the object of the feast is this: There is one God and in this one God there are three divine Persons; the Father is God, the Son is God, the Holy Spirit is God. 
Yet there are not three Gods, but one, eternal, incomprehensible God!

The Father is not more God than the Son, neither is the Son more God than the Holy Spirit. The Father is the first divine Person; the Son is the second divine Person, begotten from the nature of the Father from eternity; the Holy Spirit is the third divine Person, proceeding from the Father and the Son.

No mortal can fully fathom this sublime truth. But submit humbly and say: Lord, I believe, help my weak faith. This feast, which falls on the first Sunday after Pentecost, should make us mindful that actually every Sunday  is devoted to the honor of the Most Holy Trinity, that every Sunday is sanctified and consecrated to the triune God.

Sunday after Sunday we should recall in a spirit of gratitude the gifts which the Blessed Trinity is bestowing upon us. The Father created and predestined us; on the first day of the week He began the work of creation.

The Son redeemed us; Sunday is the "Day of the Lord," the day of His resurrection. The Holy Spirit sanctified us, made us His temple; on Sunday the Holy Spirit descended upon the infant Church. Sunday, therefore, is the day of the Most Holy Trinity.

Prayers to the Blessed Trinity

The making of the sign of the cross, which professes faith both in the redemption of Christ and in the Trinity, was practiced from the earliest centuries. St. Augustine (431) mentioned and described it many times in his sermons and letters. 

In those days Christians made the sign of the cross (Redemption) with three fingers (Trinity) on their foreheads. The words "In the name of the Father and the Son and the Holy Spirit" were added later. Almost two hundred years before Augustine, in the third century, Tertullian had already reported this touching and beautiful early Christian practice:

In all our undertakings  - when we enter a place or leave it; 
before we dress; before we bathe; when we take our meals; 
when we light the lamps in the evening; 
before we retire at night; when we sit down to read; 
before each new task--we trace the sign of the cross on our foreheads. 
(Handbook of Christian Feasts and Customs, by Francis X. Weiser)

"Glory be to the Father, and to the Son, and to the Holy Spirit, as it was in the beginning, is now, and ever shall be, world without end. Amen." 

Surely no prayer is said more often.  It forms the conclusion to every psalm, and every Hour of the Divine Office is begun with it. Truly the "Glory be" is like a chime in the church tower that is ever ringing.