The first Gospel, in the old rite, for to-day is:
18 et accedens Jesus locutus est eis dicens data est mihi omnis potestas in cælo et in terraAt the time of the legalisation of christianity in the roman empire, the chief heresy that the Catholic Church vied with was Arianism. Saint Athanasius was the chief defender, promoter and evangelist of orthodoxy. His creed, as well as the Nicene Creed, was a formulaic confession, that stated that the Father, Son and Holy Spirit were of the same substance. The Arians stated that the Son, Jesus, was a created being, therefore not equal, co-substantial and co-eternal with the Father.
19 euntes ergo docete omnes gentes baptizantes eos in nomine Patris et Filii et Spiritus Sancti
20 docentes eos servare omnia quæcumque mandavi vobis et ecce ego vobiscum sum omnibus diebus usque ad consummationem sæculi
18 And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth.
19 Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.
20 Teaching them to observe all things whatsoever I have commanded you: and behold I am with you all days, even to the consummation of the world. — Matthew xxviii
The Trinity is not part of natural theology, it cannot be ascertained by reason alone. It is revealed theology. The Revelation came on that first Pentecost. The eastern churches view Pentecost as Trinity Sunday. It has been a universal feast, in the Latin rite, since the early 14th century.
In the vernacular rite this year, cycle A, the Gospel begins with the passage that had , at one time been so favored at sporting events:
16 For God so loved the world, as to give his only begotten Son; that whosoever believeth in him, may not perish, but may have life everlasting.The last Gospel read in the old rite is not the usual beginning of the Book of John, but a portion of the sixth chapter of Luke:
17 For God sent not his Son into the world, to judge the world, but that the world may be saved by him.
18 He that believeth in him is not judged. But he that doth not believe, is already judged: because he believeth not in the name of the only begotten Son of God. — John iii.
36 Be ye therefore merciful, as your Father also is merciful.
37 Judge not, and you shall not be judged. Condemn not, and you shall not be condemned. Forgive, and you shall be forgiven.
38 Give, and it shall be given to you: good measure and pressed down and shaken together and running over shall they give into your bosom. For with the same measure that you shall mete withal, it shall be measured to you again.
39 And he spoke also to them a similitude: Can the blind lead the blind? do they not both fall into the ditch?
40 The disciple is not above his master: but every one shall be perfect, if he be as his master.
41 And why seest thou the mote in thy brother's eye: but the beam that is in thy own eye thou considerest not?
42 Or how canst thou say to thy brother: Brother, let me pull the mote out of thy eye, when thou thyself seest not the beam in thy own eye? Hypocrite, cast first the beam out of thy own eye; and then shalt thou see clearly to take out the mote from thy brother's eye.