From ‘The Prescription Against Heretics’ by Tertullian.
There is impunity in erring, if there is no delinquency; although indeed to err it is itself an act of delinquency. With impunity, I repeat, does a man ramble, when he (purposely) deserts nothing. But yet, if I have believed what I was bound to believe, and then afterwards think that there is something new to be sought after, I of course expect that there is something else to be found, although I should by no means entertain such expectation, unless it were because I either had not believed, although I apparently had become a believer, or else have ceased to believe. If I thus desert my faith, I am found to be a denier thereof. Once for all I would say, No man seeks, except him who either never possessed, or else has lost (what he sought). The old woman (in the Gospel) had lost one of her ten pieces of silver, and therefore she sought it; when, however, she found it, she ceased to look for it. The neighbour was without bread, and therefore he knocked; but as soon as the door was opened to him, and he received the bread, he discontinued knocking. The widow kept asking to be heard by the judge, because she was not admitted; but when her suit was heard, thenceforth she was silent. So that there is a limit both to seeking, and to knocking, and to asking. “For to every one that asketh,” says He, “it shall be given, and to him that knocketh it shall be opened, and by him that seeketh it shall be found.” Away with the man who is ever seeking because he never finds; for he seeks there where nothing can be found. Away with him who is always knocking because it will never be opened to him; for he knocks where there is none (to open). Away with him who is always asking because he will never be heard; for he asks of one who does not hear.
As for us, although we must still seek, and that always, yet where ought our search to be made? Amongst the heretics, where all things are foreign and opposed to our own verity, and to whom we are forbidden to draw near? What slave looks for food from a stranger, not to say an enemy of his master? What soldier expects to get bounty and pay from kings who are unallied, I might almost say hostile—unless forsooth he be a deserter, and a runaway, and a rebel? Even that old woman searched for the piece of silver within her own house. It was also at his neighbour’s door that the persevering assailant kept knocking. Nor was it to a hostile judge, although a severe one, that the widow made her appeal. No man gets instruction from that which tends to destruction. No man receives illumination from a quarter where all is darkness. Let our “seeking,” therefore be in that which is our own, and from those who are our own: and concerning that which is our own,—that, and only that, which can become an object of inquiry without impairing the rule of faith.