In the laymen sense when we say God is omniscient we say, “God knows everything”. It may be fine to use that definition when speaking to the ordinary person, but philosophically we can run into paradoxes and perhaps contradictions if we don’t clarify what we mean when we say, “Omniscience”.
In essence, to say that God is all knowing is to say that: God knows all/every true proposition(s) and does not believe any false propositions or that God knows all/every proposition(s) that is/are false.
Now the immediate objections is, “Well so what? That makes God into some sort of computer that knows a bunch of facts.” This objection is simply off the mark. In virtue of God knowing all true propositions, it follows that God perfectly knows the past, present, future, and hypotheticals. And don’t forget it also follows that God would know all possibilities. This is far from a trivial or boring definition of the word, “Omniscient”.