Visible Unity

On the eve of Christ's Passion, Jesus prayed to the Heavenly Father for his disciples, "that they may all be one...so that the world may believe that you have sent me" (Jn 17:21). What kind of unity is Jesus praying for and why? Is this unity only an invisible bond between believers, or is it something more, something both invisible and visible? Drawing upon the Scriptures, the early Church Fathers, the saints, and various popes and councils, this thesis will clarify these fundamental questions from a historical, theological, and evangelical perspective.