There are no vices more dangerous than those which simulate virtue.

Desiderius Erasmus (1465-1536) Dutch humanist philosopher and scholar

The Handbook of the Christian Soldier [Enchiridion Militis Christiani], sec. 32b (1501) [tr. Fantazzi (1989)]
    (Source)

Alternate translation:

No sins are more dangerous than those which have the appearance of virtue.
[tr. Himelick (1963), ch. 14]