But merely to possess virtue as you would an art is not enough, unless you apply it. For an art, even if unused, can still be retained in the form of theoretical knowledge, but virtue depends entirely upon its use.

[Nec vero habere virtutem satis est quasi artem aliquam, nisi utare; etsi ars quidem, cum ea non utare, scientia tamen ipsa teneri potest, virtus in usu sui tota posita est.]

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

De Re Publica [On the Republic, On the Commonwealth], Book 1, ch. 2 / sec. 2 (1.2) (54-51 BC) [tr. Sabine/Smith (1929)]
    (Source)

(Source (Latin)). Alternate translations:

Yet to possess virtue, like some art, without exercising it, is insufficient. Art indeed, when not effective, is still comprehended in science. The efficacy of all virtue consists in its use.
[tr. Featherstonhaugh (1829)]

Nor is it sufficient to possess this virtue as if it were some kind of art, unless we put it in practice. An art, indeed, though not exercised, may still be retained in knowledge; but virtue consists wholly in its proper use and action.
[tr. Yonge (1853)]

It is not enough to possess virtue, as though it were an art, unless we use it. For although, if you do not practice an art, you may yet retain it theoretically, the whole of virtue is centered in the exercise of virtue.
[ed. Harbottle (1906)]

But it is not enough to possess virtue, as if it were an art of some sort, unless you make use of it. Though it is true that an art, even if you never use it, can still remain in your possession by the very fact of your knowledge of it, yet the existence of virtue depends entirely upon its use.
[tr. Keyes (1928)]

Yet it is not enough to possess moral excellence as a kind of skill, unless you put it into practice. You can have a skill simply by knowing how to practise it, even if you never do; whereas moral excellence is entirely a matter of practice.
[tr. Rudd (1998)]

Furthermore, virtue is not some kind of knowledge to be possessed without using it: even if the intellectual possession of knowledge can be maintained without use, virtue consists entirely in its employment.
[tr. Zetzel (1999)]

Truly it is not enough to have virtue, as if it wer some sort of art, unless you use it. In fact, even if an art can be grasped by knowledge itself without using it, virtue depends wholly upon its use.
[tr. Fott (2014)]

It is not enough simply to possess virtue, as though it were a skill, unless you use it. Even a skill can be maintained through disuse by knowledge itself, but the entirety of virtue consists of its use.
[tr. Robinson (2016)]