What was the very important and necessary task that lay ahead for the disciples?

Matthew 28:19+20 recalls Jesus' command to his followers: "...go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."(Matthew 24:14)(Isaiah 52:7/Ephesians 6:15/Romans 10:15)(Luke 4:43/Luke 8:1)