Details:
The priest makes the sign of the Cross while he begins to read the Introit, which is usually taken from a Psalm. This evolved from the practice of singing a full Psalm, interspersed with the antiphon, during the entrance of the clergy, before the Prayers at the Foot of the Altar were added to the Mass in medieval times. This is indicated by the very name of "Introit".
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This image is in the public domain.
