The Church's Year
This festival is celebrated on the Sunday after Pentecost, because as soon as the apostles were instructed and consoled by the Holy Ghost, they began to preach openly that which Christ had taught them.
Why do we celebrate this festival?
That we may openly profess our faith in the doctrine of the Holy Trinity, which is the first of Christian truths, the foundation of the Christian religion, and the most sublime of all mysteries; and that we may render thanks, to the Father for having created us, to the Son for having redeemed us, and to the Holy Ghost for having sanctified us.
In praise and honor of the most Holy Trinity, the Church sings at the Introit of this day's Mass:
INTROIT Blessed be the holy Trinity and undivided Unity: we will give glory to him, because he hath shown his mercy to us: (Tob. XII.) O Lord, our Lord, how wonderful is thy name in all the earth! (Ps. VIII. 1.) Glory be to the Father and to the Son, and to the Holy Ghost, as it was in the beginning, is now, and ever shall be, world without end. Amen.
COLLECT Almighty, everlasting God, who hast granted to Thy servants, in the confession of the true faith, to acknowledge the glory of the eternal Trinity, and in the power of Thy, majesty, to adore the Unity: grant that, by steadfastness in the same faith, we may ever be defended from all adversities. Through our Lord Jesus Christ Thy Son, who liveth and reigneth with Thee, in the Unity of the Holy Ghost, God, world without end, Amen.
EPISTLE (ROM XI. 33-36.) O the depth of the riches of the wisdom and of the knowledge of God! How incomprehensible are his judgments, and how unsearchable his ways! For who hath known the mind of the Lord? Or who hath been his counsellor? Or who hath first given to him, and recompense shall be made him? For of him, and by him, and in him, are all things: to him be glory forever. Amen.