Although fear was turning you into a good citizen, fear is only in the short term a teacher of duty; and that unscrupulousness of yours, which never deserts you so long as you are not afraid, has turned you into a scoundrel.

[Quamquam bonum te timor faciebat, non diuturnus magister officii, improbum fecit ea, quae, dum timor abest, a te non discedit, audacia.]

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

Philippics [Philippicae; Antonian Orations], No. 2, ch. 36 / sec. 90 (2.36/2.90) (44-10-24 BC) [tr. Berry (2006)]
    (Source)

(Source (Latin)). Other translations:

However, the cause of your loyalty was fear, no lasting monitor of duty, while your worthlessness springs from that audacity which is ever present with you while you are free from fear.
[tr. King (1877)]

Fear is not a lasting teacher of duty.
[Timor non est diuturnus magister officii.]
[ed. Hoyt (1883)]

However, it was fear -- no steadfast teacher of duty -- that made you good: what made you unprincipled was that which, in the absence of fear, never departs from you, audacity.
[tr. Ker (Loeb) (1926)]

Although it was fear that was then making you a good citizen, which is never a lasting teacher of duty; your own audacity, which never departs from you as long as you are free from fear, has made you a worthless one.
[tr. Yonge (1903)]

Although fear (not a long-lasting teacher of one’s duty) was making you a decent person then, your arrogance, which never leaves you as long as fear is absent, has made you shameless.
[tr. McElduff (2011)]