This Sunday is the Octave Day of Pentecost, which is celebrated as Trinity Sunday - the great feast of God as He is in His inner life: Father, Son, and Holy Spirit. The goal of the Christian life is to behold the Triune God as He is in Himself - and it is that vision which will be truly blessed. Therefore, in this life, we ought to desire to know the Triune God - not merely as He is one, but in His threeness also.
In the Athanasian Creed, the Church professes that, "We worship one God in Trinity and Trinity in Unity, neither confounding the Persons, nor dividing the Essence." This is the greatest of the mysteries of our faith, and it is one that we should all endeavor to know as much as we are capable. The propers of this feast in some sense anticipate our act of adoration in heaven, when "we will see Him as He is." (1 Jn. 3:2)
Ant. Blessed be the Holy Trinity and undivided Unity; we shall confess Him, because He has shown us His mercy.
V. O Lord, our Lord, how admirable is Thy name in all the earth?
V. Glory be...
Ant. Blessed be God the Father, and the only-begotten Son of God, and also the Holy Spirit: because He has shown His mercy to us.
Ant. We bless the God of heaven, and before all the living we shall praise Him; because He has shown us His mercy.