This Lenten text is attributed to St Gregory the Great which makes it roughly 1500 years old. The translation is by Peter Scagnelli and it is set to ERHALT UNS, HERR as adapted by Johann Sebastian Bach.
The text is here. The sheet music can be purchased at OCP, where they have copyrighted the guitar chords. The tune itself is nonetheless public domain. It can also be sung to OLD HUNDREDTH.
A simple fake organ backing to learn the tune: