Now God knew that Abraham would hold nothing back from him, that he did truly fear God. To fear the Lord means to reverence him as sovereign, trust him implicitly, and obey him without protest. The sacrifice that pleases God is a heart broken of self-will, surrendered to God (Psalms 40:6–8; 51:17).
you have not withheld: Compare Romans 8:32, which uses the same verb (“spare”) as the Greek Old Testament here. If God gave us his most precious possession, he will surely provide everything we need.