The word “believe” is an example of a word whose meaning has been corrupted over time so that it is harder to understand the Bible’s intent when using it. Examples like, “I’m a believer now!”, “Seeing is believing!”, or choosing a ‘belief system’ are proper, but “I believe we’ve met” is more like “maybe”, and an older kid still sort of believing in santa to keep getting presents has belief weakened by doubt. Or a person may not want to think and determine for themselves what to believe so they ignore the doubts and deceive themselves with a lie they say they believe. They may even say they believe in something while actually denying it to themselves! So “believe” is supposed to be like “abide” (live by, accept as truth after objective study to overcome shallow ignorance and bias), not like “maybe” or “doubt” or self-deception.
An example of a topic that every single person ever born has some type of understanding of “believe” is gravity. From birth they are testing and learning the laws of gravity and know that it is real. They have a type of faith in it even though they can’t see it and don’t understand the science of it. Some people use faith in an idea to learn what to believe about how it works and the benefits/risks of obeying or disregarding the rules. The harm they suffer when they ignore the rules removes any doubt that gravity is real and must be obeyed. They are living by, abiding in, obeying. The more they learn, the more firm their belief and the more “maybes” and “doubts” they remove from their intent to obey. If they are confronted with a choice of leaping across a ditch or taking the long way around, they consider their options/abilities with gravity and make an informed decision about what to do. If they decide the jump is too risky they will err on the side of caution and walk the long way around. People with a more juvenile idea of believing will assume things about gravity and their abilities without learning and obeying, so they risk being harmed by their poor choices (pride makes them believe in maybes and self-deceptions even though they have no doubts that gravity exists). They may err on the side of convenience and defy gravity by assuming they can make the jump instead of determining what the odds are of making it, risking harm. They are also more likely to disrespect the laws of gravity by ignoring what they have learned, and need frequent reminders like slipping on ice or falling into the fountain at the mall because they are not paying enough attention (respect) to the rules.
Faith in an idea is what causes a person to test and prove something is true and leads to believing in it. Believing in it and seeing the success when following it and the failure when ignoring it leads to knowing the importance of obeying it. Seeing the success from obeying it leads to wondering what other faith ideas should be tested and added to the beliefs that must be obeyed. This cycle eventually creates a belief system for the person, they are a becoming a disciplined student (disciple) of it, learning what works IF they obey. So the definition of believe actually includes faithfulness, loyalty and obedience. Believing in something means living in it, abiding by it, staying true to it. The same is true for human ideas like believing in how to improve health by changing diet/fitness and Godly ideas like believing in what the Bible teaches about the sacrifice of Christ. The faith that a person starts with may come from other people for human endeavors but it comes from God for the people He is trying to attract to Himself.
People with a more juvenile idea about believing may say they believe in Jesus but actually don’t know what to believe or even actually know Him. Their idea of hope for a future with Him is like thinking they can win the lottery just because they have a ticket and they are deserving of a lucky break. They have no reason to believe their hopes will come true because they have not been studying and developing their relationship with Him (and at least attempting to live by what they have learned). This means they will have fear from doubt near the end of their physical life about what their eternity will be. Their belief is so weak and ill or uninformed it is actually unbelief and they will suffer the same eternal separation from God that all other unbelievers are doomed to. People that have studied their relationship with God enough to become disciples have enough faith to feel a certainty about what they believe (2 Corinthians 13:5). They can live free from doubt (James 1:6) with an expectation that their hopes are true, so they have less or no fear during this life or while contemplating their eternity.