Why do we care if we know rather than merely believe? Both in religion and life in general? I think that typically we appeal to an ethics. There are several ways this occurs.
The first is that we have a community notion that knowledge entails beliefs that are true and have a certain degree of justification. If I go into an employer and say I know how to program in C++ and Python and I’ve never programmed in my life then we recognize that is a lie and that it is wrong to tell that sort of lie. In a sense we recognize that in most cases to say we know when we don’t know is deceptive. This doesn’t mean it’s necessarily wrong to believe something we don’t have good reasons for or even to believe when we have reasons not to believe. It just means that when we claim knowledge we’re making a specific claim and we have a duty to make sure we’re being truthful about what we say. Given that it’s fair to ask when we actually do know something.
Most people think we have more of a duty beyond just being honest about how we use words though. It is better to know what is real than to merely believe based upon poor reasons. That is there is an intellectual duty to become informed. The reasons for this duty might vary. It may be based upon pragmatic reasoning –- an informed educated populace leads to a better society. (Certainly few of us would want to live in the ignorance of past cultures) It might be due to a duty we have towards others – either as a duty as citizens or a basic ethical duty towards our interactions with others which demands we become informed about them and our interactions with them. We could numerate numerous reasons. Ultimately though very few people think we are better ignorant than knowledgeable.