Charles Callahan is an American composer who published many accessible resources for liturgical organists. This particular prelude is available in The Art of Hymn Playing which is a collection of 250 introductions, preludes, free accompaniments and alternate harmonisations on hymn tunes.
The prelude is intended to be played prior to a service beginning or before the actual hymn (an introduction). The start of this prelude doesn't give any initial hints as to the tune until we get further into the piece. When the tune does come in, you will recognise it! I played this on the Hereford Cathedral (UK) sample set using the Hauptwerk VPO software on the Swell and Solo strings.