16 And the eleven disciples went into Galilee, unto the mountain where Jesus had appointed them. 17 And seeing them they adored: but some doubted.
Bede: When coming to His Passion the Lord had said to His disciples, “After I am risen I will go before you into Galilee” [Matt 26:32]; and the Angel said the same to the women [Matt 28:7]. Therefore the disciples obey the command of their Master. Eleven only go, for one had already perished.
18 And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth.
The Lord appeared to them in the mountain to signify, that His Body which at His Birth He had taken of the common dust of the human race, He had by His Resurrection exalted above all earthly things; and to teach the faithful that if they desire there to see the height of His Resurrection, they must endeavour here to pass from low pleasures to high desires.
Augustine: All is not written, as John confesses [John 21:25], for He had much conversation with them during forty days before His ascension, “being seen of them, and speaking unto them of the things pertaining to the kingdom of God” [Acts 1:3].
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: . . .
Bede: He who before His Passion had said, “Go not into the way of the Gentiles” [Matt 10:5], now, when rising from the dead, says, “Go and teach, all nations.”
Jerome: Observe the order of these injunctions. He bids the Apostles first to teach all nations, then to wash them with the sacrament of faith, and after faith and baptism then to teach them what things they ought to observe.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).