ORIGIN —During the first thousand years of Christianity there was no special feast of the Trinity, but Pope Alexander II (1073) declared every day to be devoted to the Sacred Trinity. From the ninth century on, Frankish bishops promoted a special feast of the Holy Trinity, usually on the Sunday after Pentecost. They used a Mass text by Abbot Alcuin (804).
In 1334 Pope John XXII accepted the festival into the calendar of the Western Church on the Sunday after Pentecost. A new Mass text was written. The Preface of the Trinity we hear today is the same one that appeared in the Sacramentary of St. Gregory the Great. Most of the other prayers are of later origin.
SIGN OF THE CROSS — Saint Augustine (431) mentioned it many times. Christians made the sign of the cross (Redemption) with three fingers (Trinity) on their foreheads. The words (“In the name of the Father and the Son and the Holy Spirit”) were added later. Almost two hundred years before Augustine, in the third century, Tertullian had already reported this touching and beautiful early Christian practice:
In all our undertakings — when we enter a place or leave it; before we dress; before we bathe; when we take our meals; when we light the lamps in the evening; before we retire at night; when we sit down to read; before each new task — we trace the sign of the cross on our foreheads.
DOXOLOGY — “Glory be to the Father, and to the Son, and to the Holy Spirit” was used in the Eastern Church. The second part (“as it was in the beginning...”) was added at the time of the Emperor Constantine. During the fifth century this beautiful prayer came into the Western Church and spread very quickly. Since then it has been in constant use.
SYMBOLS — During the first centuries of the Christian era the Holy Trinity was sometimes represented in painting by three young men of identical shape and looks. By the sixth century, only the Father and Son were shown in human form, the Holy Spirit represented by a dove.
In medieval times there were many imaginative pictures and designs. The Church has not officially accepted any of them, has tolerated some, forbidden others. One of the best-known symbols of this kind is the shamrock. A second plant is the pansy (viola tricolor), which even today is called “Trinity flower” in many parts of Europe. In Puerto Rico a delicately perfumed white flower with three petals is called Trinitaria. Another symbol is the figure of a triangle (Trinity) surrounded by rays (divinity) with the picture of an eye inside the triangle (omniscience and providence). This design became very popular and may be found all over Europe in homes, on wayside shrines, and even in churches. An interesting version of this symbol may be seen in the Great Seal of the United States (reproduced on every one-dollar bill).
From: Handbook of Christian Feasts and Customs by Francis X. Weiser, S.J., Harcourt, Brace and Company, New York, 1958