Abraham married Hagar and begot Ishmael. Galatians 4 says that "the one of the maidservant was born according to the flesh..from Mount Sinai, bringing forth children unto slavery, which is Hagar. Now this Hagar is Sinai the mountain in Arabia.." (vv. 23-25). In other words, Hagar represents the law. What is the law? The law represents God's demand. The Ten Commandments represent God's demands on man. God wants this and He wants that. What does it mean to keep the law? Keeping the law means giving something to God and pleasing God.
But Galatians 3:10 says, "Cursed is everyone who does not continue in all the things written in the book of the law to do them." In other words, those who say, "I will please God," are cursed. Why are they cursed? It is because man cannot please God by himself, and he is not qualified to please God (Rom. 8:7-8). In the Bible the law is frequently mentioned in conjunction with the flesh. Romans 7 is a chapter particularly on the law. It is also a chapter particularly on the flesh. What is the flesh? Simply put, the flesh is self-effort; the flesh is the self. Whenever we try to keep the law, we are in the flesh. Whenever man tries to please God by his own effort, the law comes. A person who tries to please God with the fleshly strength is one with whom God is not pleased. This is what Hagar and Ishmael represent. Hagar represents the law, while Ishmael represents the resulting flesh.
Abraham was a believer. He tried to please God and fulfill God's goal. God wanted him to have a son, and he tried to have a son by himself. Was this not according to God's will, and did he not do it to please God? Could it be wrong? However, Paul said, "The one of the maidservant was born according to the flesh." It is true that God's will should be done. But the question is who should be the one to do His will. If we try to do His will by ourselves, the result is Ishmael. Abraham was wrong, not in his goal but in his source. His goal was to see God's promise being fulfilled, but he was wrong to fulfill it by his own strength.
Now we are clear. Not only will God reject those who do things that are not pleasing to Him. He will reject even those who do things that are pleasing to Him, but who do them according to themselves. We will not please God if we sin, and neither will we please God if we try to do good by our flesh. Whether or not we please God depends on whether the cross has done its work in dealing with our flesh and the natural life. Are we saying, "God, I cannot do anything, and I am not qualified to do anything; I can only look to You"? A person who truly believes in God is one who does not act according to his flesh. God is the Master of the work. The thing that offends God the most is usurping His place in the work. This is often where our mistake lies. We cannot believe, we cannot trust, and we cannot wait. We cannot commit everything to God. This is the root of our offense against God.
God ordained that Abraham would beget a son through Sarah. Galatians 4:23 tells us that "the one of the free woman was born through promise." The free woman was Sarah. Hagar represents the law, while Sarah represents grace. What is the difference between law and grace? Doing things by ourselves is law, while grace is God doing things for us. Simply put, grace is God doing everything for us. If we are doing it, it is not grace. Only when God is doing it for us is it grace. Grace, as defined in the Bible, is not forbearance or tolerance, nor is it doing anything by ourselves. It is something specific that God does in us. The specific work God wanted to do in Abraham was begetting Isaac through Sarah. Isaac was to be begotten of Abraham, but he was to be begotten through grace and through God's promise.