The Great Commission were the last words that Jesus spoke before leaving, thus making them His most important message.
Romans 10:13-15 New International Version (NIV)
13 for, “Everyone who calls on the name of the Lord will be saved.”
14 How, then, can they call on the one they have not believed in? And how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them? 15 And how can anyone preach unless they are sent? As it is written: “How beautiful are the feet of those who bring good news!”