St. Thomas Aquinas
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Just as Christ’s Passion has not its effect in them who are not disposed towards it as they should be, so also they do not come to glory through this sacrament who receive it unworthily. Hence Augustine, expounding the same passage, observes: ‘The sacrament is one thing, the power of the sacrament another. Many receive it from the altar . . . and by receiving’ . . . die . . . Eat, then, spiritually the heavenly ‘bread, bring innocence to the altar.’ It is no wonder, then, if those who do not keep innocence, do not secure the effect of this sacrament.”
Source: St. Thomas Aquinas, Summa Theologica