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The doctrine of divine omniscience holds that God is all-knowing, that he knows all things.
Three Kinds of Knowledge
Knowledge is traditionally divided by philosophers into three types. First, there is knowledge by acquaintance, the kind of knowledge that is involved, for example, in knowing a person. Second, there is procedural knowledge, knowledge of how to do something, such as knowledge how to juggle. Third, there is propositional knowledge, knowledge of truths.
The kind of knowledge involved in omniscience is propositional knowledge; to be omniscient God must know all truths. This means that it is possible for God to be omniscient without him having all possible knowledge of every kind. It may be that God is not personally acquainted with (i.e. does not have knowledge by acquaintance of) sin, but that he knows all truths and so is omniscient.
Foreknowledge and Free Will
Omniscience is usually, but not always, taken to entail foreknowledge, knowledge of the future. There are presumably facts about the future, truths about what is going to happen. If God knows all truths, it seems, then he must therefore know the future. This has been thought by many to be problematic, implying determinism and so undermining human freedom. If God knows what choices we are going to make, then it is impossible for us to choose any other way; our decisions are determined and all we have is an illusion of freedom.