But of all injustice, theirs is certainly of the deepest die, who make it their business to appear honest men, even whilst they are practising the greatest of villainies.

[Totius autem iniustitiae nulla capitalior quam eorum, qui tum, cum maxime fallunt, id agunt, ut viri boni esse videantur.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

De Officiis [On Duties; On Moral Duty; The Offices], Book 1, ch. 13 (1.13) / sec. 41 (44 BC) [tr. Cockman (1699)]
    (Source)

(Source (Latin)). Alternate translations:

No act of injustice is more pernicious than theirs, who while they are attempting the greatest deceit, labor to appear good men.
[tr. McCartney (1798)]

But in the whole system of villainy, none is more capital than that of the men, who, when they most deceive, so manage as that they may seem to be virtuous men.
[tr. Edmonds (1865)]

But of all forms of injustice, none is more heinous than that of the men who, while they practise fraud to the utmost of their ability, do it in such a way that they appear to be good men.
[tr. Peabody (1883)]

The most criminal injustice is that of the hypocrite who hides an act of treachery under the cloak of virtue.
[tr. Gardiner (1899)]

No iniquity is more deadly than that of those who, when they are most at fault, so behave as to seem men of integrity.
[ed. Harbottle (1906)]

But of all forms of injustice, none is more flagrant than that of the hypocrite who, at the very moment when he is most false, makes it his business to appear virtuous.
[tr. Miller (1913)]

Taking all forms of injustice into account, none is more deadly than that practiced by people who act as if they are good men when they are being most treacherous.
[tr. Edinger (1974)]