Evil relates to hell because everything evil comes from hell. Faith relates to heaven because everything true that is related to faith comes from heaven. As long as we are alive in the world, we are kept walking midway between heaven and hell in the spiritual equilibrium engendered by our own choice. Hell is under our feet and heaven is over our head. whatever comes up from hell is evil and false. Whatever comes down from heaven is good and true. Because we are between these two opposites and spiritual equilibrium, we are freely able to choose, adopt, and incorporate into ourselves either the one or the other. If we choose evil and falsity, we unite ourselves to hell. If we choose goodness and truth, we unite ourselves to heaven.
The statements just made show not only that evil relates to hell and faith relates to heaven but also that evil and faith cannot be together in one object or one human being. If evil and faith were together, they would tear us apart as if we had been bound with two ropes, one of which was pulling us up and the other down. We would be like someone suspended in the air. We would be flying like a blackbird, now up, now down; when we were flying up, we would be worshiping God; when we were flying down, we would be worshiping the Devil. Anyone can see that this would be profane. No one can serve two lords without hating one and loving the other, as the Lord teaches in Matthew 6:24.
Various comparisons can illustrate the fact that there is no faith where evil exists. For example, evil is like fire. Hellfire is nothing but a love for evil. Evil consumes faith as if it were straw, and reduces it and everything related to it to ash.
Evil dwells in pitch darkness. Faith dwells in light. Evil uses falsities to extinguish faith, just as pitch darkness extinguishes light. Evil is as black as ink. Faith is as white as snow and as clear as water. Evil blackens faith the way ink blackens snow or water.
Here are other comparisons: evil cannot be united to truth that is related to faith any more than a rotten smell can be an ingredient in a sweet perfume, or urine can be an ingredient in a flavorful wine. Evil and faith cannot coexist any more than a reeking corpse and a living person can share the same bed. Evil and faith cannot live with each other any more than a wolf can live in a sheepfold, a hawk in a dovecote, or a fox in a henhouse.
from True Christianity, Section 383