In Areopagitica, Milton talks heavily about the topic of food and eating because it was something that was familiar to all his readers. Not only does everyone eat, but in the Bible that all his good Christian readers would have read there is much said about what foods are right or wrong to eat. Milton uses passages in the Bible to support his analogy that if man is free to choose what is right or wrong to consume in order to nourish his physical body, why should he not be able to judge what books should nourish him mentally?
As said above, Milton is saying that like eating, man should be able to choose what he reads. In the Old Testament, God gave Moses and Aaron a precise list of what animals they could or couldn't eat. In Acts 10.13,15, the apostle Peter has a vision where God tells him, "Rise, Peter; Kill, and eat...Do not call anything impure that God has made clean." God has come to tell Peter that all animals that he has put on the Earth are here for man to eat. In Areopagitica, there is a passage where Dionysius Alexandrinus, a bishop from the third century, is berated by a priest about "[his] venture...among those defiling volumes" (210). A vision from God told him, "Read any books what ever come to thy hands, [you are] sufficient both to judge aright, and to examine each matter" (210-11). Dionysius Alexandrinus was told by God that books are here to read. He has entrusted man with the "gift or reason to be his own chooser" (213).
Milton does not believe everything that the analogies imply. Man has been given reason, but each man uses this gift differently. "Best books to a naughty mind are not unappliable to occasions of evil" (211). To an evil man a book about evil will fill his brain with even more vile ideas. A man like that does not practice temperance and should not be reading books of that nature. Men of pure minds, on the other hand, can read any book without being corrupted. Milton thinks that one is pure if he has knowledge of evil and rejects it, like The Lady did in Comus. Books written on evil topics can't hurt a man "if the will and conscience be not defil'd" (211). These thoughts are very different then the ones found in the New Testament. "It is not what enters into the mouth that defiles the man, but what proceeds out of [it]" (Matthew 15.11). Here Jesus is saying that it doesn't matter what a man eats because it "passes through the stomach, and is eliminated" (Matthew 15.17). Whether the food is good or not makes no difference because it is your actions that prove if you are a good man. Unlike the knowledge in books that is retained by the mind, food just passes through ones body until it is gone. Reading bad books may make a man do bad things, but eating bad food will do nothing except upset ones stomach, not make him commit evil deeds.