If you live in a well-functioning society, and you have a mostly happy and successful life, you should probably go along believing whatever you currently do, because it seems to be working for you. You might want to question things if your society is dysfunctional, your life is unhappy and unsuccessful, the authorities disagree with each other, there isn't consensus in your community, you have a gut feeling that something is wrong, you find the belief system to be illogical, you find evidence to contract your beliefs, and you're predictions are consistently wrong. No wonder so many people are doubting their beliefs right now.
If we take the skeptic's view we would demand all belief systems prove themselves by evidence, logic, and the scientific method. This works fine for scientific beliefs and is an excellent way to advance scientific knowledge, but doesn't work for moral/religious beliefs, because no religious belief can be proven through logic, evidence, and science. The skeptic concludes that no religious belief is justified, but not having a belief system will drive most people insane, so we fill in the gaps with something, anything, even if the new belief system is harmful. Just because something can't be proven true doesn't mean we should reject it. It's more important that our beliefs be practically beneficial; to improve our lives and create well-functioning societies.
1. It Makes Me Happy / Keeps Me From Going Crazy
We should definitely avoid any belief system that makes us insane, depressed, or otherwise unable to live our lives successfully. This also disqualifies a lack of a belief system for most people. We should not let anyone sow doubts in a belief if they can not suggest an alternative belief system that at least fulfills the first criteria. Being "correct" isn't worth being miserable. Though we do need to be careful that we aren't choosing delusion over heartache in the short term which would make us better off in the long term. For example, if we had a cheating spouse and we lived in denial about it, admitting the truth would make us unhappy in the short term, but in the long term, it would make us happier to fix the situation.
2. It Makes Me A Better Person / It Makes Society Better
We can automatically disregard any belief system that creates dysfunction in the individual and society that holds it. We should aim for living successful lives, which is best facilitated within successful societies. So when looking for what belief system is the best, we should look at the quality of life for those practicing that belief system. The more correct a set of beliefs are, the better off the holders of those beliefs should be.
3. Everyone Else Is Doing It / Tradition
While this may not sound like a good reason to believe something and bring up the phrase "If all your friends jumped off a bridge would you do it too?", it is often the true reason why people believe things most of the time. Most of us have an innate desire to fit in with the tribe and to do, say, and believe what the tribe believes. Our ancestors' lives depended on being part of a tribe because being alone often meant death. We should also consider that if we lived in a well-functioning society that it may actually be a very good reason. After all, all of your ancestors were successful, they managed to reproduce before dying. If we do what they do we are likely to be successful as well. The problem arises if our culture has changed, everyone else is doing something harmful, and our society is dysfunctional, then we might want to question popular beliefs, especially if what's popular currently goes against the traditional beliefs of our ancestors.
4. The Authorities Say So
This may or may not be a good reason, it all depends on whether or not the authorities are wise and trustworthy. It feels compelling to believe our authority figures, but we must ask if those authority figures have skin in the game. Will they personally suffer any consequences if they are wrong? If not, I would be skeptical. Have they proven past success, wisdom, and trustworthiness? If not, I would be skeptical. Getting a fancy degree or title may sound nice, but often even those institutions that give out those degrees and titles are corrupt and incompetent themselves.
5. A Gut Feeling
In our age of reason, it's an unpopular idea to say you believe something just because you have a gut feeling. In our culture, you are taught to distrust your gut. But I trust it, I think this is a good reason to believe something, or at least to start questioning something. If I have a gut feeling that something is wrong, I look into what might be wrong instead of just suppressing it. Our subconscious minds pick up on things that our conscious minds don't, and emotions are the only way our subconscious can communicate with our conscious minds.
6. Divine Revelation
This can be a very compelling reason for the person receiving the divine revelation, but it's not a very good reason to believe someone else. Especially considering how many divine revelations contradict each other, we must be skeptical about other people's claims.
7. Logical Reasoning
Logical reasoning is a very popular reason to believe something nowadays. But the problem is that most of us are not trained in formal logic. It's very easy to trick someone into thinking that an argument follows logically, but it actually doesn't. And even if an argument is logical, it doesn't mean that our underlying assumptions are true. So I would be careful with logical reasoning, it's better for disproving things as being illogical, than it is for proving things true. Because when someone points out that something isn't logical, you can see it. But if someone says something is logical it may not be, and you just haven't noticed the flaw yet.
8. Evidence / Scientific Experiments
Evidence is a great reason to believe something if the evidence is valid. The problem is that evidence can be easily faked. Whether a study has a design flaw or the scientist just made up the data, or the findings are being misinterpreted and reported, there are lots of ways to fool people. This is why duplicating randomly controlled experiments are so important. We need a system where experimenters log their intention to do a study with an authority before doing the experiment so that the authority can follow up with them and make sure they report the results, no matter the outcome. Too often a scientist has a theory and performs an experiment and if he doesn't get the results he likes he just doesn't report his findings. We also need to systematically repeat past experiments to validate that we get the same results. The best evidence can be gleaned from research reviews where they look at many studies on the topic, from multiple different researchers. But too often the funder of a study determines the outcomes, so we should never trust studies that contradict our own eyes.
9. Accurate Predictions
This could be a good reason to believe something, but it could easily be faked through confirmation bias. If someone makes a lot of predictions and only 1% come true, people will focus on the 1% and not the 99%. If the prediction is vague they could easily bump that up to 10%. We could design an experiment where predictions are scientifically logged and measured. For example, we could determine what the best way to predict the weather is by testing different weather predicting algorithms. This is another way to prove a belief wrong. If a belief is that the world will end on a certain date and it doesn't happen that should prove that belief wrong. If we often find ourselves surprised in life, because we were making faulty predictions, we should question our belief system.
10. Persuasive Arguments
This is a terrible reason to believe something but unfortunately is the most common reason. Something can sound good, it can be nice to hear, and it's easy to believe things that are repeated often, with passion and conviction and self-righteous anger. But that doesn't make it true, it just means the speaker is skilled. We must be careful that we shield ourselves from liars, who are smooth talkers and can make persuasive arguments, but whose ideas can be destructive to us. This is how a once-functional society can become dysfunctional. The people listened to the bad but persuasive arguments for why the society should change their beliefs and way of doing things. It may have sounded like a good idea, but the results are less pleasant. This is why it's important to try new ideas on a small scale first, and see what happens over a long period of time, before adopting them widely.
In conclusion, how should we determine what we should believe? Well, we should believe whatever makes us and our society more functional. And we can prove things to be false if it makes us dysfunctional, make us unhappy long-term, if it's illogical, against good evidence, and fails at predictions. We should do a gut check and remind ourselves that just because everyone else is doing it or the authorities say so, or there are good-sounding arguments, that doesn't make it true.
New belief systems should be carefully tested in a small group of people, we should look at their demographic statistics to find out if they are living long, successful lives. Can people form a society around these shared beliefs that is stable and long-lasting? If not, it's best to steer clear.