Prudence means practical common sense, taking the trouble to think out what you are doing and what is likely to come of it. Nowadays most people hardly think of Prudence as one of the “virtues.” In fact, because Christ said we could only get into His world by being like children, many Christians have the idea that, provided you are “good,” it does not matter being a fool. But that is a misunderstanding. In the first place, most children show plenty of “prudence” about doing the things they are really interested in, and think them out quite sensibly. In the second place, as St. Paul points out, Christ never meant that we were to remain children in intelligence: on the contrary, He told us to be not only “as harmless as doves,” but also “as wise as serpents.” He wants a child’s heart, but a grown-up’s head. He wants us to be simple, single-minded, affectionate, and teachable, as good children are; but He also wants every bit of intelligence we have to be alert at its job, and in first-class fighting trim. The fact that you are giving money to a charity does not mean that you need not try to find out whether that charity is a fraud or not. The fact that what you are thinking about is God Himself (for example, when you are praying) does not mean that you can be content with the same babyish ideas which you had when you were a five-year-old. It is, of course, quite true that God will not love you any the less, or have less use for you, if you happen to have been born with a very second-rate brain. He has room for people with very little sense, but He wants every one to use what sense they have. The proper motto is not “Be good, sweet maid, and let who can be clever,” but “Be good, sweet maid, and don’t forget that this involves being as clever as you can.” God is no fonder of intellectual slackers than of any other slackers. If you are thinking of becoming a Christian, I warn you you are embarking on something which is going to take the whole of you, brains and all. But, fortunately, it works the other way round. Anyone who is honestly trying to be a Christian will soon find his intelligence being sharpened: one of the reasons why it needs no special education to be a Christian is that Christianity is an education itself. That is why an uneducated believer like Bunyan was able to write a book that has astonished the whole world.
–C.S. Lewis, Mere Christianity (my emphasis)