"Thomas," sang Carl Edwards, "look at the wounds on my body."

"Lord Jesus . . . I believe you," sang Kenneth Nafziger in reply.

These members of the Theater Wagon company were singing in medieval Latin, with melodies at least eight centuries old, and their performance last night at the National Shrine of the Immaculate Conception gave the audience (or rather the congregation) an opportunity to see something that is usually found only in textbooks. It was theater--really, opera--being born out of the collective, liturgical worship of medieval Christendom.

The brief music drama "Peregrinus," composed in the 12th century for the Cathedral of Beauvais, was presented as it was intended to be--in the context of an evening prayer service in a large church, with processionals, Gregorian psalms and antiphons, the Magnificat and the Lord's Prayer. It is hard to imagine such an effective presentation except on a special occasion such as the international symposium on Gregorian chant now taking place at Catholic University.

"Peregrinus" is less show-biz oriented than the familiar "Play of Daniel," also from Beauvais. Based on the appearances of Christ to his disciples after the resurrection, it is a brief drama of grief and consolation, doubt transformed into belief. Its effect was close to that of prayer--precisely as it should be.