Gaudens gaudebo in Domino, et exsultabit anima mea in Deo meo: quia induit me vestimentis salutis, et indumento justitiae circumdedit me, quasi sponsam ornatam monilibus suis.
“I will joyfully rejoice in the Lord, and my soul will exult in my God, for he has clothed me with vestmens of salvation, and he robed me with a dress of justice, like a bride, ornamented with necklaces.” (cf. Is 61:10)
Click here to listen to an audio of the chant, sung by Liborius Lumma, Innsbruck (Austria).
This melody was copied from Vocem iucunditatis, but of course there are no original medieval neumes for this piece—the Solemnity of the Immaculate Conception is much younger.
The dogma of Immaculate Conception is a burden in ecumenical dialogue; no church except the Roman Catholic teaches it.
If we try to make this Catholic dogma understandable, Gaudens gaudebo can lead in a good direction: Mary’s election can be regarded as a festive vestment that was given to her without personal merits. And Gal 3:27 says: “For all of you who were baptized into Christ have clothed yourselves with Christ.”
If we combine both of these ideas we can conclude that this solemnity is not about something miraculous happening biologically between Mary’s mother and father. It rather shows us Mary as an image or an archetype of Christianity. And that is something that one hopes other Christian denominations can agree to, even if they (for good biblical and historical reasons) literally deny the Roman Catholic dogma that this solemnity relies on.