I think one way for Christians to overcome the hesitation of becoming missional and venturing "out there" versus remaining overly content staying within the boundaries of the Church is to move away from the traditional dualistic view of salvation... the sense that it's "us versus them".
When we believe that ours is the only way to attain eternal Oneness with God, then we automatically create division within humanity. It is this perception of division, I feel, that pushes us toward "our kind" and encourages us to keep away from "them".
We can only be truly missional by emptying ourselves completely, always looking outward beyond our safe borders... and never seeing the differences, never judging. It's not up to us anyway.