Ask a child if it’s hard to believe in Santa Claus. It’s not at all if people you trust keep telling you a lie that this being is real, and praise you and give presents when you accept their nonsense.
The claim of atheists is that religion is simply wrong in its metaphysical claims. It is wrong to claim that a theistic God exists as represented in religious scriptures, that the soul exists, that angels exist, that humans descended from Adam and Eve, that God created life by fiat during an intense 6 day period of magic, that Jesus was born of a virgin, that he rose from the dead, and that an afterlife exists.
If I let myself believe anything on insufficient evidence, there may be no great harm done by the mere belief; it may be true after all, or I may never have occasion to exhibit it in outward acts. But I cannot help doing this great wrong towards Man, that I make myself credulous. The danger to society is not merely that it should believe wrong things, though that is great enough; but that it should become credulous, and lose the habit of testing things and inquiring into them; for then it must sink back into savagery.
Science is not a “belief system” but a process and methodology for seeking an objective reality. Of course because scientific exploration is a human endeavor it comes with all the flaws of humanity: ego, short-sightedness, corruption and greed. But unlike a “belief system” such as religion untethered to an objective truth, science is over time self-policing; competing scientists have a strong incentive to corroborate and build on the findings of others; but equally, to prove other scientists wrong by means that can be duplicated by others.