St. Alphonsus Liguori on False Appearances of Holiness

[I thought this was funny:]

But some will say, "But this man tutors my daughter; he is a saint." The saints are in Heaven, but the saints that are on earth are flesh, and by proximate occasions, they may become devils.

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Source: St. Alphonsus Liguori, "Advice to Parents," http://www.olrl.org/snt_docs/advice.shtml.