Dionysius Alexandrinus was about the year 240, a person of great name in the Church for piety and learning, who had wont to avail himself much against hereticks by being conversant in their Books; untill a certain Presbyter laid it scrupulously to his conscience, how he durst venture himselfe among those defiling volumes. The worthy man loath to give offence fell into a new debate with himselfe what was to be thought; when suddenly a vision sent from God, it is his own Epistle that so averrs it, confirm'd him in these words: Read any books what ever come to thy hands, for thou art sufficient both to judge aright, and to examine each matter. To this revelation he assented the sooner, as he confesses, because it was answerable to that of the Apostle to the Thessalonians, Prove all things, hold fast that which is good. And he might have added another remarkable saying of the same Author; To the pure, all things are pure, not only meats and drinks, but all kinde of knowledge whether of good or evill; the knowledge cannot defile, nor consequently the books, if the will and conscience be not defil'd. For books are as meats and viands are; some of good, some of evill substance; and yet God in that unapocryphall vision, said without exception, Rise Peter, kill and eat, leaving the choice to each mans discretion. Wholesome meats to a vitiated stomack differ little or nothing from unwholesome; and best books to a naughty mind are not unappliable to occasions of evill. Bad meats will scarce breed good nourishment in the healthiest concoction; but herein the difference is of bad books, that they to a discreet and judicious Reader serve in many respects to discover, to confute, to forewarn, and to illustrate.
(John Milton, AREOPAGITICA; part 2)