From The New Interpreter’s Bible: Genesis to Leviticus:

Brueggemann notes correctly that this test “is not a game with God; God genuinely does not know…. The flow of the narrative accomplishes something in the awareness of God. He did not know. Now he knows” (Brueggeman, Genesis, 187). The test is as real for God as it is for Abraham.

The test is not designed to teach Abraham something—that he is too attached to Isaac, or that Isaac is “pure gift,” or that he must learn to cling to God rather than to the content of the promise. Experience always teaches, of course, and Abraham certainly learns. But nowhere does the text say that he now trusts more in God or has learned a lesson of some sort. Rather, the test confirms a fact: Abraham trusts deeply that God has his best interests at heart so that he will follow where God’s command leads (a point repeated in vv. 12 and 16). The only one said to learn anything from the test is God: “Now I know” (v. 12). God does not teach; rather, God learns. For the sake of the future, God needs to know about Abraham’s trust.

While God knew what was likely to happen, God does not have absolute certainty as to how Abraham would respond. God has in view the larger divine purpose, not just divine curiosity or an internal divine need. The story addresses a future that encompasses all the families of the earth: Is Abraham the faithful one who can carry that purpose along? Or does God need to take some other course of action, perhaps even look for another?

Is the promise of God thereby made conditional? In some sense, yes (see. vv. 16-18). Fidelity was not optional. God could not have used a disloyal Abraham for the purposes God intends.