Although some like to use poetic beginning phrasing when explaining the concept to the public, scientists don’t’ actually claim the big bang was the beginning of existence. Only as far back as our current level of physics suggests. Even then James Web Telescope has found some problems with our current models which suggest we’ll need to modify them. There are other problems too with dark energy and dark matter. No universal theory to connect quantum mechanics to gravity And so on.
Scientists aren’t sages. They aren’t magicians. Nor do they claim to be. They use evidence to come up with theories then test those theories to see how well they stand up. Comparing science to religion is absurd. Without scientists we’d still be living the stone age. Religion is essentially static. Some ancient cult charlatan made up a story that’s not true, then subsequent cult members keep believing the same nonsense even when can validate its untrue (e.g there is academic consensus evolution happened not unsupported myths of Adam and Eve).
Unlike religion, science evolves as new information and better theories to explain existence becomes available. When the evidence warrants it, scientists say they are wrong. Religion is the antithesis of science.