Blesssed be the Holy Trinity and undivided Unity: we will give glory to Him, because He has shown His mercy to us! (Introit [Tob 12:6] Trinity Sunday)
At our Baptism the entire Trinity takes possession of our entire being (Gospel); hence our lifelong necessity of practical true faith in the presence of the Trinity in our souls, as our defense against "all misfortunes" (Prayer); the necessity of being ever conscious in our prayers that we are the adopted "sons of the Father," "co-heirs with Christ," and "temples of the Holy Ghost"; the necessity of shunning mortal sin, which automatically exiles the Trinity from our souls.
The offering of the Mass to the Trinity (Introit, Offertory) makes the offering of our lives (Secret, Communion) acceptable and pleasing. This offering of the Mass and of ourselves brings Divine help to body and soul (Postcommunion).
NOTES ON THE HOLY TRINITY:
The dogma of the Most Holy Trinity, the Father, Son and Holy Ghost is contained throughout the liturgy. Sunday is the special day of the Trinity. God the Father created the world on the first day of the week. God the Son rose from the dead on a Sunday morning. The Holy Ghost descended upon the Apostles on Pentecost Sunday.
Since 1334 the feast day has replaced the proper Mass of the I Sunday After Pentecost. It was formerly used as the Mass for the ordinations of priests on the evening of the Ember Saturday which was formerly done as a vigil. The displaced Mass is said on days having no feasts, if any, during the coming week, but sadly is otherwise is almost forgotten but should be heard on Tuesday and Wednesday as these are 'Feria' days in 2015.