Among the issues that have divided Eastern and Western Christians throughout the centuries, few have had as long and interesting a history as the question of the filioque. Christians everywhere confess their faith in the ancient words of the Nicene Creed. But rather than serve as a source of unity, the Creed has been one of the chief sources of division, as East and West profess their faith in the Trinitarian God using different language. In the Orthodox East, the faithful profess their belief in "the Holy Spirit, who proceeds from the Father." In the West, however, they say they believe in...