A good friend of mine pointed out this Sunday (23 June) Roman Catholics (RC) and the Revised Common Lectionary (RCL) have a different Gospel Reading. Usually, the Gospel Reading is the same. Why not this Sunday?
There is no official explanation to go with either lectionary. So I am intuiting the reason for the change (RCL is a revision of RC) as much as you are.
In Ordinary Time we essentially work through a synoptic gospel semicontinuously. Generally, if a story is told in more than one gospel, that is an opportunity to tell that story in one year, but it is not normally in all three years*.
This Sunday RC read Luke 9:18-24 – Peter’s confession; Jesus foretells his passion; if any would come after me…
This Sunday RCL read Luke 8:26-39 – The Gerasene Demoniac.
Both stories occur in all three synoptic gospels.
The story of Peter’s confession occurs in each year in the RC Lectionary [Year A Ordinary 21; Year B Ordinary 24; and this Year C Ordinary 12]. No prizes for guessing why! The story of the Gerasene Demoniac never occurs in the RC Lectionary. I have no explanation of why it is omitted there.
RCL has replaced one of the three occurrences of the story of Peter’s confession with the Lukan version of the Gerasene Demoniac, whilst retaining the Matthew and Markan versions of Peter’s confession in the other two years. This appears, at least to me, to fit better with the principles that undergird the lectionary.
*I am well aware of the issues surrounding this practice of skipping a story, and of the understanding that the same story may function differently in different gospels, what I am describing is the RC and RCL actual practice.