Reason and experience are transcendental and metaphysical.
According to Kant, Pure Reason isn't what it seems. Without experience, reason is unfounded and useless. Without reason, experience can't be made sense of. This means that there must be a practice of discerning the truth about the unseen patterns and principles of the world, through experience and reason together, in harmony.
He says that this is possible because reason and experience share something in common—we experience them a priori. He argues that the world must be metaphysical, because math translates into reality, for instance, and because by assuming the world is structured, we can operate in the world. This utilitarian mode is held in tension with the transcendental truth of experience, that reality extends beyond the scope of human understanding. Part of why Kant expresses such a distaste for religion is that people oversimplify the truth instead of allowing the world to be as complicated as it really is.
To understand "God" is not easy.
The idea of God interacting with the world is one that was taken for granted historically, explained in theory by religion, but untreated by logic. Kant is attempting to show that—even if there is a God—what we can observe about the universe through observation and contemplation is different than what religious people mean by "God."
Part of why Kant wrote this was to break down a philosophical view of the metaphysical world into a hierarchy, in an attempt to illustrate how seriously complicated and daunting the task is of making sense of the world really is—especially without accepting a superstitious view of God.
The priority of reason over inference.
One of Kant's issues with the history of philosophy that he belongs to is that historically, people's beliefs have been created by superstitious inferences about the world (religion) instead of through the difficult process of parsing the data for one's self. He explains that true knowledge is the product of true reason, and he dismisses religious ideas as fundamentally unhelpful, because religious ideas often make rational claims about transcendental principles. This isn't just an issue with theology by the way—many people hold beliefs that they accepted without scrutiny.
