Genesis 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Abraham responded to God’s Grace in His Covenant Promise by believing God. He obeyed the voice of the Lord because he believed. Abraham’s obedience to God’s voice and the carrying out of His impossible ask arises out of his God-created faith in the Promise. The Lord said, ”All nations will be blessed in your seed.” So how could he now kill the seed? Abraham did not know why God told him to do it, nor did he know how He was going to pull this off. But “God said it; I believe it; that’s good enough for me!” Abraham could not obey the Voice without complete trust in the Lord. He did not obey to gain God’s favor; he obeyed because he already had it.
So also it is with us: we believe and love God so that we do good works according to His will. We obey His Word because we have already received His favor and blessing. Outward obedience is the logical and natural evidence of inward faith. Because we have received Grace, therefore we obey.
Now one might say: it was easy for Abraham because he heard God’s voice directly, but He doesn’t speak to me like that. Oh, but He does: the Holy Spirit speaks directly to our hearts through the Gospel in the preached and written Word and in the visible Word of the Sacraments. We hear the voice of God (even though it appears to come from a human). We believe the Gospel. Then we obey the Word.