This is a good point, and holds in the majority of cases, although there are other considerations which should also be mentioned.

Since all maps are 'flawed' by definition, an important question is whether the flaws in your map actually interact with your goals, and if they do whether they are beneficial or harmful. It's usually not a good use of your energy to fine tune areas of your map which don't have any impact on your life and actively wasteful to "fix" them in ways which make it harder to achieve your goals.

Incorrect beliefs can be useful in the aggregate even if they fail in certain situations, as long as those situations are rare or inconsequential enough. I can be utterly wrong in my belief that there are no tigers in New York City (there are several in the Bronx Zoo, not to mention that more might well be kept illegally as pets) but it's completely orthogonal to my daily life and thus not important enough to spend effort investigating. And if I had a pathological fear of tigers, I would gain a pretty significant advantage from that same false belief; I would do well to maintain it even if presented with genuine counter-evidence.

I think that most religions are wrong to harmful degrees, but it's not an ironclad rule of rationality that beliefs must be maximally accurate. A pessimist is actually more accurate in their assessments of people, but optimists are happier and more successful; if your rationality insists you cannot be optimistic, then it is not useful and should be ignored.

having the belief of conservative Christian leads you to acts that are inefficient for reaching your goals

I would submit that it rather depends on your goals.

I agree that having a correct map of reality is needed if you care about arriving at some (I hate this word) "materialist" goal, but not everyone can live in a more liberal area of the US/world. Areas where not fitting into the local community creates more burdens than necessary.

For example, when I was in the military, I identified as an atheist pretty openly, barely concealing my contempt for religion. One of my last Enlisted Performance Reports, my supervisor said I would do better if I were a Christian. I wasn't sure if that was a threat, or him implying that I was immoral and thus not quite a fit in the military community, but nevertheless I was penalized (in his eyes) for not being religious.

There's the map of reality, the map of the physical roads and buildings, but there's also the map of human interactions. One can signal consciously, but what I gather is that most people signal unconsciously and then generate a cached version of themselves that eventually becomes the real them (belief in belief and all that). While I might say I had an accurate map of reality when I was in the military, I didn't have, nor did I want to even accept, that there was another map of social situations that would also have helped me reach my goals more efficiently if I had an accurate map of it.

Hmm, its true that your map needs to be correct (in the sense that it corresponds to reality) in order to reach your destination, but it need not be wholly so.

Let's say I'm a firm believer that I must obey the laws written on a sheet of paper somewhere. I think they were written there by all powerful alien forces, obedience to whose dictates is the sole criteria for determining virtue. Any evidence to the contrary (say, the fact that no one has seen the sheet of paper) is part of the alien's test. Could you work with me?

It depends on what I think is on the paper.

If I believe that something approaching your, I dunno, call it ethics, are written on there, then it doesn't really matter to you too much why this is the case. The two of us ought to be able to cooperate (you holding your nose at my fundamentalism, me rolling my eyes at your relativism.).

My map of reality is wildly inaccurate at the edge of my neighborhood, but the two of us are only going to the drug store (voting on an issue we agree on).

I guess what I'm trying to say is that having a flawed map of a city only means you won't reach your goal if the flaws manifest themselves on the route between you and your goal. Flying Saucer Cult members still tie their shoes just fine (maybe, if you think they don't please substitute a task you believe that they routinely accomplish, perhaps donning their cult attire).

I believe that what you say about caring about truth as an instrumental value to reach my goal is true IFF my goal is essentially discovery-related. I need an accurate map when I venture beyond my neighborhood. But what if my goal is pretty much local?

I have a brother who is happily married, works to support his family, regular church goer, roots for his local sports teams, etc. He's pleasant and friendly to his many friends and associates, well respected. He learned what he needs to know for his job long ago, and is uninterested in improving at it (there's not much room to do so).

What does he need truth for? He is at his destination, his map was sufficient to reach it. How can truth improve his attending of church, or his time at the bar with his buds, or help him play with his kids? Its superfluous to his life.