Jesus gave his famous sermon on the Beatitudes on the mountain. This has religious importance. Moses received the Torah on a mountain. Jesus proclaimed his new teaching on a mountain. Now the commissioning of the disciples takes place on a mountain. The mountain is the place of God’s divine revelation. Still, after all the disciples have seen, heard and experienced, some had doubts. Matthew wants us to know that doubting is part of faith. This was true for the disciples (v.17), and it is true for us.
Now the disciples are to accept a universal commission to evangelise all nations, Gentiles and Jews included. They are to baptise everyone in the name of the Trinity, and finally, they are to teach and model what Jesus taught them, especially in the Sermon on the Mount. The source of their strength to accomplish this will be the presence of Jesus himself. He promises to be with the disciples until the end of time.
This commissioning is aimed at all believing Christians throughout the ages, including ourselves. We are to fulfil this commissioning not only by the words we speak, but especially by the way we live our lives. It was in Galilee that it all started (Mt 4:12). It was there that the disciples first heard the call (Mt 4:15) and it was there that Jesus promised to reunite them again after the resurrection (Mt 26:31). For Matthew, the proclamation starts in Galilee with the pagans (Mt 4:15) in order to herald the passage from the Jews to the pagans (meaning "us").
A universal call to teach "all," means a universal call to "all" to listen and hear the message of Jesus through his disciples. It is, therefore, an invitation to a deeper relationship with the Father, but that can only happen by listening to and accepting the message of Jesus. His word is Truth and Life!