16Now the eleven disciples went to Galilee, to the mountain to which Jesus had directed them. 17When they saw him, they worshiped him; but some doubted. 18And Jesus came and said to them, “All authority in heaven and on earth has been given to me. 19Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20and teaching them to obey everything that I have commanded you. And remember, I am with you always, to the end of the age.”
Translation: And the eleven disciples journeyed into the Galilee, into the mountain where Jesus had directed them. And seeing him, they worshiped, but they doubted. And Jesus came (and) spoke to them, saying, "All power has been given to me in heaven and upon the earth. As you have gone, therefore, teach all the nations, baptizing them into the name of the Father and of the Son and of the Holy Ghost, teaching them to keep all as I commanded you. And behold! I am with you all the days until the consummation of the eternal."
Background and situation: These are the closing words of Matthew's gospel. As is characteristic for Matthew's gospel, Jesus is again presented as Moses-like, yet greater-than Moses. Like Moses, he speaks on a mountain and gives commandments. As greater-than Moses, he now commands "all authority."
The Holy Trinity: This is the gospel reading for Holy Trinity Sunday, the only day of the church year said to be dedicated specifically to a doctrinal position.
In reality, of course, all church holidays are about some aspect of doctrine. Christmas, for example, is not at all the "birthday of Jesus," but a holy day of the church dedicated to the incarnation, a doctrinal teaching which counters the heresies of gnosticism and docetism.
The lection is likely chosen because of its reference to "the name of the Father and of the Son and of the Holy Spirit." The saying is anachronistic for the time of Jesus, who, bereft of expensive seminary education, would likely have been thoroughly mystified by something called "trinitarian theology."