Atheism can mean anything from actively believing gods do not exist to passively not believing gods exist. Most atheists believe gods exist only as ideas in human minds. Some atheists believe the very idea of a god is incoherent.
Atheism is a natural answer to the big questions of life. Most atheists believe the universe is as it seems to be when we apply reason and science to the evidence of our senses. We do not need to invent gods to explain either what we know or what we do not yet know.
Religions typically have creeds supposedly revealed by supernatural beings, while atheists form our own beliefs. Atheist groups claim to speak only on behalf of our members, not on behalf of the creator of the universe.
Atheism is not of itself a belief system. There are as many specific atheistic belief systems as there are atheists. But two significant general beliefs necessarily follow from atheism: that reality is not revealed by gods, and that morality does not come from gods. These are significant general beliefs in a world where most people believe the opposite.
Faith is belief that is disproportionate to the best currently available evidence. Atheism is proportionate to the evidence. There is no reasonable evidence that gods exist, and a lot of evidence that humans invented the idea.
With regard to certainty, many atheists are also agnostics. Strictly speaking, we cannot be certain about anything. But we can be as certain that the Christian god does not exist as Christians are that Thor or Zeus do not exist. Give us reliable evidence and we will change our minds.
We each determine our own sense of meaning. We have evolved to notice patterns in nature, which helped previous generations to survive. This can cause us to mistakenly see patterns and ascribe agency, including to gods, that are not there.
Morality is a natural process of our brains, based on empathy, compassion, reciprocity and reason. It enables us to distinguish good moral ideas in the Bible, such as love thy neighbour, from bad moral ideas in the Bible, such as killing people for gathering sticks on the Sabbath.