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]