The most important concept in missiology is that of God becoming man. This is significant because no religion makes this claim. Before the fall, God already had a plan in place to cover man's sin (nakedness) until the advent of the Seed of the Woman who would finally take away the SIN of man by his death, resurrection, and ascension.
Man's desire to become "like God" is what lured him to rebel. Now that God, by his grace, has made it possible through his incarnational birth for man to become "like God", the kingdom of darkness, led by Satan, is alarmed and on the rampage against mankind and the Church.
That is why the Church must go to all the world and preach the Gospel. This is the third time God has given the command to go to man. This is further discussed in The Significance of the Holy Spirit in Mission Dei.
Since man failed twice before, Jesus instructed his disciples to wait in Jerusalem until they received power to go. The coming of the Holy Spirit on the Day of Pentecost marked a paradigm shift in Heilsgeschichte, or salvation history. The author stresses the irreplaceable role of the Holy Spirit in the success of Missio Dei, or the Mission of God.