The churches in the Christian world teach one God; this is common knowledge. They teach this because all their teachings come from the Word, and those teachings are all integrated as long as one God is acknowledged not only orally but also with the heart. It is common in Christianity today, however, for people to confess one God with their lips but three with their hearts. To such people God is no more than a verbal expression. To them theology as a whole is a kind of golden idol locked up in a case, and the key to unlock it is held by church leaders alone. When such people read the Word they do not detect any light in it or from it—not even the impression that there is one God. To them the Word is marred with blotches, and the oneness of God in it is obscured. The Lord described people like this in Matthew: “You will hear with your hearing but you will not understand; and you will see by seeing, but you will not perceive. They have closed their eyes to avoid ever seeing with their eyes, hearing with their ears, understanding with their hearts, turning themselves around, and being healed by me” (Matthew 13:14–15). All such Christians are like people who go into windowless rooms to avoid the light and who grope for food and coins by feeling their way along the walls. Eventually they develop eyesight like that of owls and see in the dark. They are like a woman who has many husbands—she is a promiscuous woman rather than a wife. They are like a young woman who accepts rings from multiple suitors, and after the wedding, rents out her nights not only to one of them but also to the rest.
from True Christianity, Volume 1, Section 7