They then are to be warned, whom age or imperfection hinders from the office of preaching, and yet rashness impels, lest while they hastily arrogate to themselves so responsible an office, they should cut themselves off from the way of future amendment. For the Truth Itself which could suddenly strengthen those whom it wished, in order to give an example to those that follow, that imperfect men should not presume to preach, after having fully instructed the disciples concerning the virtue of preaching, commanded them to abide in the city, until they were endued with power from on high. For we abide in a city, when we keep ourselves close within the gates of our minds, lest by speaking we wander beyond them; that when we are perfectly endued with divine power, we may then as it were go out beyond ourselves to instruct others.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Past. 3. c. 25.