Omniscience is the attribute of knowing all truths. Its scriptural expressions are pervasive and vivid: "you know my sitting down and my rising up, you understand my thought afar off" (Psalm 139:2); "even the hairs of your head are all numbered" (Matthew 10:30); "with Him are the keys of the unseen; none knows them but He... not a leaf falls but He knows it" (Qur'an 6:59).
The sharpest problem it raises is the relation of divine foreknowledge to human freedom. If God knew infallibly a thousand years ago what I will do tomorrow, it is difficult to see how I could do otherwise; and if I could not, it is difficult to see how the act is free or how I can justly be judged for it. Four families of answer recur.
The first is that God is outside time. Boethius, in the Consolation of Philosophy, and Aquinas after him, held that God knows all events in an eternal present rather than in advance, so that divine knowledge is more like observation than prediction and no more determines its object than my seeing you sit down makes you sit. Critics object that this relocates the problem rather than solving it, since eternal knowledge is still fixed.
The second is Molinism, from the sixteenth-century Jesuit Luis de Molina, which posits middle knowledge: God knows what any free creature would freely do in any possible circumstance, and can therefore providentially arrange circumstances without determining choices. The ensuing De Auxiliis controversy with the Dominicans was so bitter that Rome eventually forbade either side to call the other heretical and left the question open.
The third accepts determination. Strands of Calvinist theology and the Ash'ari tradition hold that God's decree settles what happens, and locate freedom in acting according to one's own will rather than in the ability to have done otherwise, a compatibilist move. The Ash'ari doctrine of kasb, acquisition, attempts a middle way in which God creates the act and the human being acquires it and is answerable for it; critics from the Mu'tazila onward have questioned whether the term names anything.
The fourth denies that there is anything there to be known. Open theism, developed in the 1990s by Clark Pinnock, John Sanders and Gregory Boyd, holds that future free acts are not yet determinate and that not knowing them is therefore no failure of omniscience. It has been sharply contested within evangelical theology, where its opponents regard it as a diminishment of God and its defenders regard it as the honest reading of the biblical narratives in which God responds, relents and is grieved.