An atheist believes that the available evidence disproves the theory that there is a god or gods.
Religions require us to believe "as many as six impossible things before breakfast" as the creation of one cleric happily described her expectation.
Miracles are by definition occurrences which cannot be explained by the laws of nature, science or medicine; otherwise they would not be miraculous. Yet every saint, each one hitherto a normal human, has been expected to be party to one. Gibbon, in his Decline and Fall of the Roman Empire drily remarks that the early church had men who had witnessed and performed miracles, faithfully reported as true by adherents, which they themselves had singularly failed to mention in their own writings. Such cynicism!
But miracles are but part. What solid evidence do we have, do the faithful have, for the existence of their favoured deities ? The faithful have none, as atheists see it. We are told that, if one child survives a tornado, when his siblings die in it, that that the prayers of his parents have been answered. Yet the almighty being who saved the life is the same one who did not save the lives of the others. What logic leads to that?
Religious texts, written on the authority of or conveying the words of the deity, are to be followed and every statement of anything which counters all known laws of nature is to be believed. And we are told, in some, of an afterlife, and of men who live hundreds of years, and much more besides. Yet no human has ever produced evidence which can support these assertions, any more than they can produce evidence of divine authority for not eating pork ( ignoring the common sense reason that it can be dangerous to do so)
That seems to be the difference between what an atheist believes and what the religious believe; a question of evidence to support each belief.