Vox Christi, Latin for Voice of Christ, is a setting of Jesus' words in a vocal work such as a Passion, an Oratorium or a Cantata.
Conventionally, for instance in Protestant music of the Baroque era, the vox Christi is set for a bass voice.
Jesus Christus ist um unsrer Missetat willen verwundet is a St Mark Passion which originated in Hamburg in the first decade of the 18th century.
[1] In his Brockes Passion George Frideric Handel set Jesus' words for bass in recitatives, ariosos and arias.
In Bach's extant Passion compositions the vox Christi bass combines with an Evangelist's tenor voice.