Disciples make disciples.
Multiplication is a theme throughout the Bible starting with God creating seed-bearing plants in the Garden of Eden and commanding the animals and the people to "go forth and multiply".
In the Great Commission (Matthew 28:16-20) Jesus commands the disciples to make disciples and those disciples are to obey everything Jesus commanded, so they too are to make disciples.
What is the mission?
Disciple-making
Mentoring
Communication
Listening
Teaching