John 20:21 (NIV)
I am sending you.
None of us call ourselves to anything in this life. It is God who calls us and empowers us. Some of us are called to the pulpit while others are called to ho out into the world and make disciples of all nations. The authority to do God's will comes from the Lord Himself ad dies the power. He is the One who sends us and our job is to simply obey.