Part of the ongoing Verse Quick Reference project.

Gen 22:12 And He said, “Do not lay your hand on the lad, or do anything to him; for now I know that you fear God, since you have not withheld your son, your only son, from Me.”

In Genesis 22, God begins a test of Abraham. The purpose of this test can likely be found in verse 12 and 16: God is testing Abraham to see if he would kill his only son for God. The language found in Genesis 22:12 is God seeing the results of the test and learning how Abraham would act. “Now I know” is the language employed. This makes sense in context. Kurt Williams writes:

Putting all of our Christian presuppositions aside, if we can be comfortable with a God who does not know every detail of our future decisions, would not such an interpretation actually make sense out of this whole incident of the near sacrifice of Isaac? God tested Abraham because so that God could learn something. It was a genuine discerning on God’s part to make sure that he had selected the right person for the job of creating a family that would eventually bless the world. If Abraham ended the test with a failing grade, a new plan would need to be initiated.[2] But in fact the test is passed with flying colors and so God reiterates the covenant to him in the verses that immediately follow (Genesis 22.15-20). Abraham, for a time, helped release God from the immediate bind at hand.

Due to the context and implications, this verse has led many theologians to reconsider their adherence to exhaustive divine foreknowledge. Joel S. Kaminsky writes:

So what might we learn about God from this story? I remember the moment when that dimension of the text opened up for me. My homiletics colleague, Richard Ward, and I were doing a teaching session together, and he recited Gen 22 from memory. In the freshness of that new medium, I heard a verse I had always passed over before, although I do not recall his giving it any special emphasis. Again, the angel of the LORD is speaking: “Do not stretch out your hand to the lad and do not do a single thing to him, for now I know that you are a God-fearer, and you did not withhold your son, your only one, from me” (v. 12). If we take those words seriously—and in this narrative not a word is wasted—then we have to believe that there is something God now knows for the first time. (For all its theocentricity, the book of Genesis gives little comfort to the doctrine of divine omniscience.) What God knows now is so crucial that this most terrible “test” (v. 1) was devised, in order to show whether in fact Abraham cares for God above everything and everyone else—even above Isaac, his son and his own slender hope for fulfillment of God’s promise.

I spoke earlier of cultivating generosity toward the text, if we are indeed to befriend it. Generosity toward the Old Testament must mean this at least: accepting the text on its own terms, literally, working seriously with the language it offers us. The advantage of this present reading is that it is directed by the words of the passage rather than by an extraneous idea—the immorality of child sacrifice, the omniscience of God—however valid that idea might be in another interpretive situation.

This reading also coheres with the larger narrative context, to which the very first words of the chapter point us: “After these things, God tested Abraham.” After what things? Where are we in the history of salvation? At this point, all God’s eggs are in Abraham’s basket, almost literally. Recall that after the tower of Babel, God gave up on working a blessing directly upon all humankind and adopted a new strategy: channeling the blessing through Abraham’s line (Gen 12:3). Our story takes account of that new divine strategy: “And all the nations of the earth will find blessing through your seed, because you heeded my voice” (22:18). God, having been badly and repeatedly burned by human sin throughout the first chapters of Genesis, yet still passionately desirous of working blessing in the world, now consents to become totally vulnerable on the point of this one man’s faithfulness. But the narrative has just cast a shadow of doubt over Abraham’s total faith in God. Remember those two episodes in which Abraham has Sarah pass herself off as his sister? In Egypt and again in Canaan he lets his beautiful wife go into a king’s harem, rather than trusting God to protect them on their sojourn (Gen 12:10-17 and 20:1-18). “After these things, God tested Abraham.” After all that, we can begin to understand why God must know for sure whether the single human thread upon which the blessing hangs will hold firm.
-Joel S. Kaminsky, Jews, Christians, and the Theology of the Hebrew Scriptures

The context is hard for even Calvinists to deny. John Calvin acknowledges the face value reading, but dismisses it as contradictory to his beliefs about God:

12. Now I know that thou fearest God. The exposition of Augustine, ‘I have caused thee to know,’ is forced. But how can any thing become known to God, to whom all things have always been present? Truly, by condescending to the manner of men, God here says that what he has proved by experiment, is now made known to himself. And he speaks thus with us, not according to his own infinite wisdom, but according to our infirmity.