Lessons in Truth opens from a single premise and does not let go of it: God is not a ruler positioned somewhere outside a person's life, deciding whether to intervene, but a presence already indwelling every person, waiting to be recognized rather than persuaded. Cady treats sickness, poverty and fear as conditions that follow from forgetting that presence, not as punishments handed down from outside or as the simple bad luck of an indifferent universe.
The lessons work through that premise one everyday problem at a time. Rather than staying at the level of abstract doctrine, Cady walks a reader through affirmative prayer as a practice, specific phrasing meant to be spoken or held in mind, aimed at replacing fear or lack with a clear statement of the good already understood to be present. Scripture appears throughout, but read the way Unity as a whole reads it, as metaphysical instruction about the individual's own inner state rather than as literal history to be taken at face value.
What made the book durable within the movement was not novelty of doctrine, since the indwelling God and affirmative prayer were already New Thought commonplaces by 1894, but clarity of instruction. A reader finishing Lessons in Truth has a method to try that same day, which is exactly what Cady, trained to send a patient home with something to do, seems to have intended.