Dominica Sanctissime Trinitatis: "Teach all nations"

Matt 28:18-20
At that time, Jesus said unto His disciples: All power is given unto Me in heaven and in earth. Go ye, therefore, and teach all nations, baptizing them in the Name of the Father, and of the Son, and of the Holy Ghost. And so on.

Homily by St. Gregory of Nazianzus, Patriarch of Constantinople.
Treatise on the Faith.
There is no Catholic but knoweth that the Father is a Very Father, the Son a Very Son, and the Holy Ghost a Very Holy Ghost, even as the Lord Himself saith unto His Apostles: "Go ye and baptize all nations in the Name of the Father, and of the Son, and of the Holy Ghost." This is that Perfect Trinity Who is but One being, and of Whom therefore we testify that His Substance is one. For we make no division in God, as divisions are made in bodies, but we testify, that, according to the power of the Divine Nature, Which standeth not in matter, the Persons named have a real existence, and that God is One.