Q: Is there a quick answer to this question: at the Easter Vigil, if there are no baptisms, why do we not use Psalm 42 as the response to the Ezekiel reading?
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A: In Glory in the Cross p. 139, I note that in the 8th century, Psalm 42 used to be the last reading at the Vigil, so it had some chops when baptism followed. I think the revised lectionary tried to broaden the options, keeping that psalm associated with baptisms, as a deer longs for running streams.
But the third edition of the missal, which includes the psalm citations for the first time, names only Ps 42, giving neither of the other options. It seems unconcerned if baptisms follow. This possibly represents a change in thought that the lectionary has not yet caught up to.