The day of Pentecost, when the Church celebrates the descent of the Holy Spirit upon the apostles, represents the culmination of God’s Providence in regard to mankind, the completion of the salvation of mankind from original sin, and is the apex of all the blessings that God has bestowed on His beloved creation. This day also marks the establishment of God’s Church on earth, and in this Church, until the very end of time, man can achieve salvation through the sacraments of baptism and the Eucharist. And also on this day, insofar as man’s limited mind can encompass, God reveals Himself to man in the fullness of the Trinity, and for this reason the holiday is also called Trinity Day.
The dogma of the Holy Trinity is the basic dogma of Christianity. This dogma instructs us in the truth about the triunity of the Divine Being, i.e. that God is one in Essence, but triune in Persons, and that each Person has specific characteristics: the Father has no beginning, the Son is pre-eternally born from the Father, the Holy Spirit issues from the Father. We worship the Holy Trinity as One, but at the same time we glorify all three Persons. Thus, in praying to the Trinity, the Church appeals to it in the singular and not the plural form; for example: We glorify Thee (and not you), the Father and the Son and the Holy Spirit, now and ever and unto the ages of ages, amen.
Though it is the most important of all Christian dogmas, the dogma of the Holy Trinity is at the same time the most difficult for the mind of man to grasp. How one God is in three Persons - that is a mystery which our mind is unable to fathom. However, the principle of triunity - three in one - can be seen throughout the universe, which, as God’s creation, explains to us by analogy this great mystery of its Creator’s being. Triunity is the basic principle underlying all the works of divine wisdom. A prime example is our sun. The sun is a physical object which radiates light and heat. Each of these three manifestations - light, heat, substance - are perceived by us separately, and yet all three make up a single object. We can also see an analogy, for example, in the three states of matter: solid, liquid, gaseous; in the fact that the entire spectrum of colors in the world rests on three basic colors: red, blue and yellow; in glass, which we can see, through which we can see and in which everything is reflected; in the fact that man himself is triune - composed of body, soul and spirit; and many other examples.
The Christian Church, while being conscious of the incomprehensibility of the dogma of the Trinity, sees in it a great revelation which raises the Christian faith immeasurably above any religion that simply believes in one God. The dogma of triunity indicates the fullness of internal life within God Himself, because God is love, and God’s love cannot extend only towards the world He has created, but is also turned upon the inner life of the Trinity. Thus, God is sole, but not solitary. And the love of God in Trinity pours out over the world in the most direct manner: God is above us, God is with us, God is in us and all creation. Above us is God the Father, the basis of all being, the source of all blessings, Who loves us and cares for us. With us is God the Son, Who became man out of ulimate love for us, Who sacrificed Himself for our salvation, so that through Him we could know God and become the sons of God. Within us and all creation is God the Holy Spirit, the Comforter, Who permeates all things and gives life to all things. And these three Divine Persons, Whose existence is pre-eternal, Who manifested Themselves subtly in the Old Testament (as, for example, to Abraham in the guise of three travellers), revealed Themselves openly to the world with the coming of the Son of God to earth, and on Trinity Day remain with us in Their fullness, being one power, one being, one Divinity (hymn for Pentecost).