The second source of authority is our calling from the church. A key part of that legitimizing comes from the larger community of faith that calls us to various roles of leadership. The leadership roles never become private possessions to be guarded and protected. Leadership is about service, not prerogative. The third source of a leader's authority comes from the calling of the context.
Leadership is finally about actual people in actual circumstances.
People may give us a leadership position, but the authority needed to lead must be worked out among the people with whom we serve. It is from a leader's credibility with those with whom the leader works most closely that a leader is able to gain an essential element of authority. To summarize, one might think of the sources of authority as the theological, ecclesial, and contextual. This is the wrong question. Isaiah speaks of authority resting upon the shoulders of the Messiah.