That surely depends on which sort of belief it is. If it is about something subjective, then you can be absolutely certain. You can know whether or not you believe in God (whatever you mean by God), for example. If you actually don’t, but you say you do, then you can also be sure you are lying.
Beliefs about mathematics are known to be true - 2 + 2 always equals 4. When it comes to beliefs about mind-external objective reality then in a lot of cases it is clear that we aren’t sure, but in a lot of cases we can be absolutely certain. The most controversial case is the strongest claims of the hard sciences. Water is made of hydrogen and oxygen…do we really want to say we strictly aren’t sure? I think that is absurd. We are absolutely certain about this, and the same goes for the rest of the core claims of the hard sciences.