The Hymn of Jesus:
What hymn was sung at the institution of the Lord's Supper?

In the history of Christian hymnody, the account of hymn singing as described in Matthew 26:30 is pivotal: “And when they had sung a hymn, they went out to the Mount of Olives.” This is the earliest account of hymn singing by a group of Christians and it is interesting to note that Christ himself is quite possibly the inaugurator of congregational singing [in the Christian tradition], particularly in its connection to Holy Communion. (Louis F. Benson, Hymnody of the Christian Church, p. 27).

This post-communion hymn was ritual song, and must have been so sensed by the disciples. Thus it became at once not only the precedent but the spring of our church song, which in all the main streams that have started from it continues to be ritual song. (Benson, p. 28)

With the institution of the discipline of hymnology in the 19th century, there has been much speculation about the exact hymn which may have been sung. The writer of the Apocryphal Gnostic text, The Acts of John describes the scene and discloses the “actual hymn” (Gnosticism was a heresy condemned by the early Church and these gnostic writings are not part of the official history of the church. They are presented here as an interesting historical note):

Jesus Sang:

Glory to Thee, Father! (and they moving around in a circle answered him) Amen! Glory to Thee, Charis!*Glory to Thee,Spirit! Glory to Thee,Holy One! Glory to Thy Glory! Amen! We praise Thee, O Father; We give thanks to Thee, O Light; In Whom Darkness dwells not! Amen! For what we would give thanks, I say: I would be saved; and I would save. Amen! I would be loosed; and I would loose. Amen! I would be broken; and I would break. Amen! I would be born; and I wish to give birth. Amen! I would eat; and I would be eaten. Amen! I would hear; and I would be heard. Amen! I would understand; and I would be understood. Amen! I would be washed; and I would wash. Amen!

Now Charis* is dancing.

I would play on the pipes; dance all of you, Amen! I would play a song of mourning; lament all of you, Amen! I would flee; and I would stay. Amen! I would be adorned; and I would adorn. Amen! I would be atoned; and I would at-one. Amen! I have no house; and I would have houses. Amen! I have no temple; and I have temples. Amen! I am a lamp to thee who seest me. Amen! I am a mirror to thee who understandest me. Amen! I am a door to thee who knockest at me. Amen! I am a way for thee a wayfarer. Amen! Now answer to my dancing! See thyself in me who speak; And seeing what I do, keep silence on my mysteries. Understand by dancing, What I do; For thine is the passion of man That I am to suffer. Thou couldst not all be aware of that thou does suffer. If I were not sent as the Logos by the Father. Seeing what I suffer; thou sawest me suffering; And seeing, thou didst not stand still, But wast moved greatly, thou wast moved to be wise. Thou hast me for a couch; rest thou upon me. What I am thou shalt know when I depart. what now I am seen to be, I am not. But what I am thou shalt see when thou comest- If thou hadst known how to suffer, Thou wouldst have power not to suffer. Know then how to suffer, than thou wilt have power not to suffer. That which thou knowest not, I myself will instruct thee. I am thy God, not the betrayers«. I would be kept in time with holy soul. In me know thou the Logos of Sophia. Say thou to me again: Glory to Thee, Father! Glory to Thee, Logos! Glory to Thee, Holy Spirit! But as for me, if thou wouldst know who I was: In a word I am the Logos who did dance all things, and who was not ashamed at all. It was I who danced. But do thou understand all, and understanding, say: Glory to Thee, Father! Amen! Amen! Amen!

[*The term, Charis refers to one of the Aeons in the Gnostic worldview. Aeon (Aiwn) seems to have been formed from the words Aei wn (ever-existing), meaning "an emanation from the divine substance, subsisting coordinately and co-eternally with the Deity, the Pleroma still remaining one." - Rev. Wigan Harvey, Irenaeus, cxix., 1857]

For Western Christianity, the Hymn of Jesus is usually identified as one of the Old Testament Psalms (113-118) which would have been usual for the Jewish Passover tradition and which ends with the second section of the Hallel (Psalms 115-118, Hallel-ujah). See The Last Seder.