HIPPOLYTUS: Great crimes are never single, they are link’d
To former faults. He who has once transgress’d
May violate at last all that men hold
Most sacred; vice, like virtue, has degrees
Of progress; innocence was never seen
To sink at once into the lowest depths
Of guilt.

[HIPPOLYTE: Quelques crimes toujours precedent les grands crimes.
Quiconque a pu franchir les bornes légitimes
Peut violer enfin les droits les plus sacrés ;
Ainsi que la vertu, le crime a ses degrés ;
Et jamais on n’a vu la timide innocence
Passer subitement à l’extrême licence.]

Jean Racine (1639-1699) French dramatist

Phèdre [Phædra], Act 4, sc. 2, l. 1094ff (1677-01-01) [tr. Boswell (1897)]
    (Source)

(Source (French)). Other translations:

Crime, like virtue, hath degrees; one single day can not make a bad man just; nor can the good, in such short season, pass suddenly to utter baseness.
[tr. Heron (1858), 3.1]

Some crimes always precede great crimes; whoever has overstepped the legitimate limits, may at last violate the most sacred rights; thus, as well as virtue, crime has its degrees, and we have never seen timid innocence pass suddenly into extreme licentiousness.
[tr. Mongan (1885)]

Some lesser crimes always precede great sin.
He who hath once the bounds of right transgressed
May violate the most sacred laws at last;
But even as virtue, vice hath its degrees,
And modest innocence one never sees
Pass suddenly to wanton ways and lewd.
[tr. Lockert (1936)]

A man who can transgress the lawful boundaries
may violate the most sacred rights in the end.
Like virtue, crime has its gradations;
Never has timid innocence
suddenly become extreme depravity.
[Unk.]

Crime like virtue has its degrees; and timid innocence was never known to blossom suddenly into extreme license.
[Bartlett's]