Neither do men put new wine into old wineskins: else the wineskins break, and the wine runs out, and the wineskins perish: but they put new wine into new wineskins, and both are preserved.
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Hilary of Poitiers
Figuratively, this His answer, that while the Bridegroom was present with them, His disciples needed not to fast, teaches us the joy of His presence, and the sacrament of the holy food, which none shall lack, while He is present, that is, while one keeps Christ in the eye of the mind. He says, they shall fast when He is taken away from them, because all who do not believe that Christ is risen, shall not have the food of life. For in the faith of the resurrection the sacrament of the heavenly bread is received.
By these examples He shows that neither our souls nor bodies, being so weakened by inveteracy of sin, are capable of the sacraments of the new grace.