A large portion of the population of established religions today are nominal members (i.e.  members only by name). Most of them still attend worship/church services but some never go to church at all. Most of these nominal members believe in the essential doctrines of their religion specially those that seem to be right in their judgment. But, when the teachings of their religion go against their common sense and rational thinking, they reject them.

The pure and simple belief in God -- the Uncaused Cause and Designer of all things -- without the added baggage of unreasonable and unsubstantial claims of special revelations (i.e. ancient sacred books) is what defines deism. A deist’s knowledge about God is based on reason, nature and his own conscience.