Trinity Sunday is a celebration that focuses on the foundation of the Christian faith and that man’s salvation is the work of the Father who willed it, the Son who did it and the Holy Spirit who makes it available to believers. The Eucharistic prayer always ends with the great doxology that clearly whose work salvation was and is.
Having completed the commemoration of the mysteries of our salvation, from Christ’s birth to the descent of the Holy Spirit, we now contemplate the central mystery of our faith, that is, the Most Holy Trinity—one God in three persons, the Father, the Son, and the Holy Spirit. Although the Old Testament proclaims that God is one, the New Testament explicitly mentions three distinct persons in the Godhead. In our Lord’s final instruction to his disciples, before his Ascension, he commanded them: “Go, therefore, make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit” (Matt. 28:19).
When Paul writes to his beloved Corinthian Christians, he ends his letter with the blessing: “The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all” (2 Cor. 13:13). We have been baptized in the name of these same three persons, and we profess that though they are distinct, nevertheless, they are undivided in splendor and are equal in glory and majesty (as expressed in today’s Preface). At the same time, these three persons are but one Lord, one God. The earliest reference to a feast honoring the Blessed Trinity is in Tours, in 796. The feast was later introduced at Cluny in 1091, and St. Thomas Becket adopted it at Canterbury in 1162. Pope John XXII (1316–34) approved the feast for the universal Church in 1331.
Resource:  Tylenda, J. N. (2003). Saints and Feasts of the Liturgical Year (p. 115). Washington, D.C.: Georgetown University Press.