They were to teach all nations !
Matt 28:17-20
17And when they saw him, they worshipped him: but some doubted.
18And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.
19Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
20Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen.
The people Jesus had in mind here for the Apostles to reach were unconverted,but made spiritually alive elect sinners, who would be teachable. They would be discipled.
In fact the word for Teach in vs 19 is the greek word mathēteuō and means to:
to be a disciple of one
a) to follow his precepts and instructions
2) to make a disciple
a) to teach, instruct
Christ's Power would be with them and for them, making alive all those who would be ready to be instructed. For instance, The Ethiopian Eunuch Acts 8:
26-31
26And the angel of the Lord spake unto Philip, saying, Arise, and go toward the south unto the way that goeth down from Jerusalem unto Gaza, which is desert.
27And he arose and went: and, behold, a man of Ethiopia, an eunuch of great authority under Candace queen of the Ethiopians, who had the charge of all her treasure, and had come to Jerusalem for to worship,
28Was returning, and sitting in his chariot read Esaias the prophet.
29Then the Spirit said unto Philip, Go near, and join thyself to this chariot.
30And Philip ran thither to him, and heard him read the prophet Esaias, and said, Understandest thou what thou readest?
31And he said, How can I, except some man should guide me? And he desired Philip that he would come up and sit with him.
Their mission was very specific, it would be to those whom God would give a Spiritual Desire to, to be instructed.
It began on the day of Pentecost after the Spirit came for that very witnessing Purpose. Those who Peter preached to were awaken by the Spirit and desired to be discipled, instructed , thats why they asked Acts 2:37
37Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
They had been made willing to be instructed in the Day of Christ's Power Ps110:3Thy people shall be willing in the day of thy power,
So this commission was to a very specific prepared people. Like John the Baptist Ministry was unto Lk 1:17
And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord.
The Holy Spirit made them ready in the Apostles case !
Matt 28:18-19 was for the regenerated but yet unconverted elect to be spiritually instructed !