There is knowledge, and there is opinion. To know something means to understand its causes and effects. To hold belief or opinion means to ascribe to certain understandings without having proved them for oneself; if having done so, these understandings become things that are known.
No one disputes the Law of Gravity. Even infants figure out its validity, meaning they prove it to themselves. It existed before we did, and will exist afterward. Anyone who would dispute that would be deemed crazy, or cutting edge physicist. Even there, the physicist would have to qualify his or her argument; that argument would only be found valid if tested under the Scientific Process, which is the method by which humans know things.
We believe a lot of things. By undertaking the hard work of being human, by reasoning, we can come to know them. Certainly our knowledge is incomplete, but that does not imply it is invalid.
There is an objective reality of cause and effect in which we exist. To negate that is to apply mysticism to things already known. Certainly mysticism can (and has) been applied to things we do not know, but reason drives us forward and makes the unknown comprehensible. To go in reverse is for humankind to devolve.