A friend told me God was not really omniscient because he didn't know where Adam and Eve was after the fall. He had to ask where they were. My friend referred to Genesis 3.9. I laughed and told him how ridiculous his understanding was.
I hoped God had answered my friend directly, telling him something like: "Can a man hide himself in hiding places so I do not see him?" declares the LORD "Do I not fill the heavens and the earth?" declares the LORD. [Jeremiah 23.14]
Psalm 139 says he knows all about us. He declares the end from the beginning [Isaiah 46.10] and that alone tells volumes about God's omniscience. Moreover, just look at Revelation. He knows how everything will come to an end. But why did he have to test Abraham to know his heart?
In fact, why do we all have to receive Jesus and be born again? God knows right from the start who would receive and who would reject Jesus and his salvation by grace. So why not just judge us all according to that foreknowledge? Why wait for all of us to actually be born again or to reject salvation? We don't even have to be born biologically and live our lives on earth just so God can judge us. He can judge us even before our birth because he has foreknowledge of how we would live our lives.
But then he willed that we live our lives on earth and come to a point of receiving (or rejecting) Jesus as Savior, not so he'd know, but to give us a chance to know why he'd judge us accordingly. The same with Abraham. He knew Abraham's heart (primarily, being his Creator), but he had to allow Abraham to decide and exercise his free will.
God has this preference or proclivity of expressing it as if he doesn't know ("Now I know"), but he does it so the subject (Abraham) and bible readers would understand the judgment. In this case, Abraham knew (and we, the readers know) why God chose him because "you have not withheld your only son." But for that result to come about, Abraham had to be tested.
So, now we know one factor why God would choose certain individuals---and that is, if we do not withhold from him even our loved ones, or anyone or anything we hold dear to us or we think we can't live without. Without this test on Abraham, we'd have no clue about these things. God says, "Now I know" to really mean "Now you know."
GOD's Flesh E-Book:
To buy, email me at email@example.com.
GOD's Flesh: "...yet in my flesh I will see God." [Job 19.26-17].