The scene depicted is the moment when the three Apostles, Peter, James and John sleep while Christ keeps his prayer vigil.  In the background Jerusalem is depicted with a procession of figures led by Judas Iscariot, who is about to betray Christ to the authorities. The Angel appears in the sky with the chalice and the Host.  The vignette shows the Mass of St. Gregory, with his triple barred cross, unique to Popes Gregory and Sylvester. The story is taken from a moment in the Mass, when St. Gregory was faced with an unbeliever in the congregation, whom he managed to convert through the aid of prayer.

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