The reason for this is that goodwill without faith is not goodwill, and faith without goodwill is not faith, as I have shown above, Sections 355-358. Goodwill does not exist all alone by itself, and neither does faith. As a result, it cannot be said that goodwill produces any good works on its own or that faith produces any good works on its own.
The situation is similar with the will and the intellect. There is no such thing as a will that exists all alone by itself; It would not produce anything. There is no such thing as an intellect that exists all alone by itself; it would not produce anything either. All productivity comes from both faculties working together; it comes from the intellect in connection with the will. This similarity exists because the will is the home of goodwill and the intellect is the home of faith.
I said, “even less does faith alone produce them,” because faith is truth. To live our faith is to put truths into action. Truths enlighten goodwill and the practice of it. The Lord teaches that truths are enlightening when he says, “Those who do the truth come to the light so their works will be revealed, since those works were done in God” (John 3:21). Therefore when we follow truths in our doing of good works, we do good works “in the light,” meaning intelligently and wisely.
The partnership between goodwill and faith is like the marriage between a husband and a wife. All their physical offspring are born to both the husband as their father and the wife as their mother. Likewise, all our spiritual offspring are born to goodwill as their father and faith as their mother. Spiritual offspring are concepts of goodness and truth. These concepts allow us to recognize the lineage of whole spiritual families. In fact, in the Word’s spiritual meaning “a husband” and “a father” refer to goodness related to goodwill, and “a wife” and “a mother” refer to truth related to faith.
From these parallels it is again clear that goodwill by itself or faith by itself could not produce good works, just as a husband by himself or a wife by herself could not produce children.
The truths that relate to faith not only enlighten goodwill, they also enhance its quality and even nourish it. Therefore if we have goodwill but we have no truths related to faith, we are like someone walking in a garden at night, plucking pieces of fruit from the trees without knowing whether they are beneficial or harmful to eat. Since the truths related to faith not only enlighten goodwill but also enhance its quality, as I said, it follows that goodwill without truths that related to faith is like pieces of fruit without any juice in them, like parched figs or like grapes after the wine has been pressed out of them.
Since truths nourish faith, as I also said, it follows that if goodwill lacks truths that are related to faith, that goodwill has no more nourishment than we would have from eating a piece of burnt toast and drinking filthy water from a pond.
from True Christianity, Section 377