Lactantius — Divine Institutes 4.23
Whoever gives precepts to humans and shapes others' life and manners--I ask whether he ought himself to do what he commands or not. If he has not done it, the precepts are loosed. For if the things commanded are good and place human life in the best state, the teacher must not segregate himself from the number and company of the humans among whom he acts, and he must live in the same way he teaches others to live, lest if he lives otherwise he himself pull faith from his precepts and make his teaching lighter by dissolving in deed what he tries to bind in words. Everyone who hears a commander does not want the necessity of obeying imposed as if liberty were taken away. So he answers the teacher: I cannot do what you order, since they are impossible. You forbid me to be angry, to desire, to be moved by lust, to fear pain and death--but this is so against nature that all animals are subject to these affections. Or if you think nature can be resisted so far, you yourself who command, do it, so I may know it can be done. But when you yourself do not do it, what insolence is it to want to impose on a free human laws you yourself do not obey? First learn, you who teach, and before you correct others' manners, correct your own.
Who would deny this reply is just? Indeed such a teacher will come into contempt and be mocked in turn, because he will seem himself to be mocking. What then will that teacher do if these things have been set against him? How will he take away the excuse of the stubborn, unless by present deeds he teaches that he teaches things that are possible? Hence it comes about that no one obeys the philosophers' precepts. For humans prefer examples to words, because speaking is easy and performing hard--and would that as many did well as speak well! But those who command and do not do are without faith: if they are humans they will be despised as light; if God, the excuse of human frailty will be set against him. It remains that words be confirmed by deeds--which philosophers cannot do. So when the teachers themselves are conquered by the affections they proclaim ought to be conquered, they can educate no one to the virtue they falsely proclaim, and for that cause they think no one has yet existed as a perfect wise man--that is, one in whom highest teaching and knowledge agreed with highest virtue and perfect justice; which indeed was true. For since the world was made no such person has existed except Christ, who both handed on wisdom by word and confirmed the teaching by present virtue.
Latin
Quicumque praecepta dat hominibus ad uitam moresque fingit aliorum, quaero debeatne ipse facere quae praecipit an non debeat. si non fecerit, soluta praecepta sunt. si enim bona sunt quae praecipiuntur, si uitam hominum in optimo statu collocant, non se debet ipse praeceptor a numero coetuque hominum segregare inter quos agit, et ipsi eodem modo uiuendum est quo docet esse uiuendum, ne si aliter uixerit, ipse praeceptis suis fidem detrahat leuioremque doctrinam suam faciat, si re ipsa resoluat quod uerbis nitatur adstringere. unus quisque enim cum audit praecipientem, non uult inponi sibi necessitatem parendi, tamquam sibi ius libertatis adimatur. respondet itaque doctori hoc modo: \'non possum facere quae iubes, quoniam sunt inpossibilia. uetas me irasci, uetas cupere, uetas libidine commoueri, uetas dolorem, uetas mortem timere, sed hoc adeo contra naturam est, ut his adfectibus animalia uniuersa subiecta sint: uel si adeo putas repugnari posse naturae, tu ipse qui praecipis fac, ut sciam fieri posse. cum autem ipse non facias, quae insolentia est ut homini libero inponere uelis leges quibus ipse non pareas? prius disce qui doces et antequam mores aliorum corrigas, tuos corrige.\' quis neget iustam esse hanc responsionem? quin etiam in contemtum ueniet huiusmodi doctor et deludetur uicissim, quia uidebitur et ipse deludere.
quid ergo faciet ille praeceptor, si haec ei fuerint obposita? quomodo adimet excusationem contumacibus, nisi ut praesentibus factis doceat possibilia se docere? inde euenit ut philosophorum praeceptis nullus obtemperet. homines enim malunt exempla quam uerba, quia loqui facile est, praestare difficile: utinamque tam multi bene facerent quam multi locuntur bene! sed qui praecipiunt nec faciunt, abest ab iis fides: et si homines fuerint, contemnentur ut leues, si deus, opponetur ei excusatio fragilitatis humanae. superest ut factis uerba firmentur: quod philosophi facere nequeunt. itaque cum ipsi praeceptores uincantur adfectibus quos uinci praedicant oportere, neminem possunt ad uirtutem quam falso praedicant erudire ob eamque causam putant neminem adhuc perfectum extitisse sapientem, id est in quo summae doctrinae ac scientiae summa uirtus et perfecta iustitia consenserit; quod quidem uerum fuit.
nemo enim post mundum conditum talis extitit nisi Christus, qui et uerbo sapientiam tradidit et doctrinam praesenti uirtute firmauit.