The principle is

"The principle is that we are to be in the world, but not of the world, like a ship should be in the water, but water shouldn’t be in the ship! But if the world is influencing us, it is clear we are unequally yoked together with unbelievers–[it] may come through a book, a movie–or even through worldly friends–Christians are far too indiscriminate about the things they allow to influence their minds and lives."–David Guzik.

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