Friends, in today’s Gospel, Jesus prays: “Holy Father, keep them in your name that you have given me, so that they may be one just as we are one.”
The Church is one because its founder is one. Jesus compels a choice precisely because he claims to speak and act in the very person of God. Jesus simply cannot be one teacher among many, and therefore those who walk in his way must be exclusively with him.
Moreover, the God whom Jesus incarnates is one. The Israelite conception of God is fiercely monotheistic and hence it excludes any diversity or syncretism at the level of basic belief: “The Lord—‘Jealous’ his name—is a jealous God.” Joseph Ratzinger commented that the opening line of the Nicene Creed, Credo in unum Deum (I believe in one God), is a subversive statement, because it automatically rules out any rival claimant to ultimate concern.
To say that one accepts only the God of Israel and Jesus Christ is to say that one rejects as ultimate any human being, any culture, any political party, any artistic form, or any set of ideas. A Christian is someone who, at the most fundamental level of his or her being, is centered on the one God of Jesus Christ.