1 Corinthians 5:6

Your glorying is not good. — There is possibly a reference here to some boasting regarding their spiritual state contained in the letter which had reached St. Paul from Corinth, and to which part of this Epistle is a reply. (See 1 Corinthians 7:1.) So long as there is that one bad person amongst you it gives a bad character to the whole community, as leaven, though it may not have pervaded the entire lump, still makes it not the unleavened bread which was necessary for the Paschal Feast. This Epistle being written shortly before Pentecost (1 Corinthians 16:8), it was very likely some time about or soon after Easter, hence the leaven and the Paschal Feast naturally suggest themselves as illustrations. The Apostle passes on rapidly from the mention of the leaven to the whole scene of the feast. As with the most minute and scrupulous care the Jew would remove every atom of leaven when the Paschal lamb was to be eaten, so our Paschal Lamb having been slain, we must take care that no moral leaven remains in the sacred household of the Church while she keeps her perpetual feast of prayer and thanksgiving.