The solemnity of the Most Holy Trinity is ancient. The origins of the solemnity can be traced to the seventh century. It has been on the Church's universal calendar for nearly seven hundred years. "The central mystery of Christian faith and life" is the mystery of the Most Blessed Trinity (CCC, 234 - Click Here to read more). This solemnity offers us time to meditate upon this defining mystery of our faith.
This is also the first of the three doctrinal feasts celebrated after Pentecost, which commemorate not a person but a theological tenet, doctrine, or devotion. Today's solemnity celebrates the doctrine of the Triune God, one God in three divine persons - Father, Son, and Holy Spirit.
In the fourth century, the Church underwent a serious crisis when Arius, a priest of Alexandria, denied the divinity of Christ, and as a consequence, faith in the Trinity and the equality of the three divine Persons. The heresy, called Arianism, was condemned by the councils of Nicaea (325) and Constantinople (381). The result was the formulation of the Creed that we recite at Mass today.
In celebration of the Eucharist, notice how frequently we articulate our belief in the Trinity: with the sign of the Cross, the Doxology at the end of the presidential prayers, the Gloria, the Profession of Faith, and the Eucharistic Prayer culminating in the Great Doxology above, and finally with the Solemn Blessing at the end of Mass.
The stained glass window is from St. Thomas the Apostle Church in Bloomfield, NJ.