People who believe things easily without having to be convinced are credulous. Sales people are always hoping that someone credulous picks up the phone during a sales call.
Credulous comes from the 16th-century Latin credulus, or "easily believes." A synonym for credulous is gullible, and both terms describe a person who accepts something willingly without a lot of supporting facts. Calling someone credulous can imply that the person is naive and simple. An individual isn't necessarily insulted by being called credulous, though, because some objects of belief, like religions and unicorns, come with a willing leap of faith for believing in what is unseen.