In this sermon, the preacher discusses the concept of believers having two natures - an old nature and a new nature. He explains that before conversion, the old nature controls a person's actions, often leading to unkind behavior. However, when someone is born again, they receive a new nature, which leads to a battle between the old and new natures. The preacher emphasizes the importance of making a decision to yield to the new nature and live a righteous life. He also mentions the story of Isaac and Ishmael as an illustration of the conflict between the two natures.

Now, last time we saw in chapter 20 that Abraham and Sarah had to make confession and put away that sin, which was a habit that went back to an agreement they'd made 25 years before that wherever they went, that Sarah would say that Abraham was her brother and not her husband. And they had two very unfortunate experiences. One was in Egypt, and this one in the 20th chapter. But this sin must be dealt with, confessed, and put away before Isaac could be born. Now in chapter 21, we have the birth of Isaac. Will you listen? And the Lord visited Sarah as he had said, and the Lord did unto Sarah as he had spoken. For Sarah conceived and bare Abraham a son in his old age at the set time of which God had spoken to him. Now you'll notice that there is a very striking similarity between the birth of Isaac and the birth of Christ. And I believe that the birth of Isaac is given to us for that reason, to set mankind before Christ came this great truth. Isaac was born at the set time God had promised. And Paul says in Galatians 4, 4, that in the fullness of time, God sent forth his son, born of a woman born under the law. Now I'm reading on here in Genesis 21, verse 3. And Abraham called the name of his son that was born unto him, whom Sarah bared to him, Isaac. And Abraham circumcised his son Isaac, being eight days old, as God had commanded him. And Abraham was a hundred years old when his son Isaac was born unto him. And Sarah said, God hath made me to laugh, so that all that hear will laugh with me. And she said, Who would have said unto Abraham that Sarah should have given children suck? For I have born him a son in his old age. And the child grew and was weaned. And Abraham made a great feast the same day that Isaac was weaned. And Sarah saw the son of Hagar the Egyptian, which was born unto Abraham, mocking. Wherefore she said unto Abraham, Cast out this bondwoman and her son. For the son of this bondwoman shall not be heir with my son, even with Isaac. And the thing was very grievous in Abraham's sight because of his son. Now I break off the reading there at verse 11 because here there are some very remarkable truths that we need to lay hold of. First of all, the birth of Isaac was a miraculous birth. It was contrary to nature. In the fourth chapter of Romans, Abraham did not count his own body, which was dead, nor yet the deadness of Sarah's womb. Now out of death, God brings forth life. Now this is a miraculous birth. And we need to call attention to this, that God did not flash on the world the birth of Christ, the supernatural birth of Christ as being something new. He began to prepare men for it. And so way back here at the birth of Isaac, you have a miraculous birth. Now we find also here that God had to deal with both of these, both Sarah and with Abraham. And we saw that in the last chapter. God had to deal with them. They had to recognize now that they could do nothing, that it would be impossible for them. Abraham's 100 years old, Sarah's 90 years old. In other words, the birth of Isaac must be a birth that they have nothing really to do with. Now the coming of this little boy Isaac into the home sure did produce a great deal of difficulty. Fact of the matter is, we find that this boy that was the son of Hagar, Ishmael, he was mocking. And we begin now to see the nature and the character of this boy Ishmael. Up to this point, he seems to be a pretty nice boy. But now with the appearance of this other son in the family, he really shows his true colors. That is an illustration, by the way, of the fact that a believer has two natures. Now until you're converted, you've got an old nature, and that old nature controls you. Do what you want to do. Or as the popular secular song has it, doing what comes naturally. And what man does that comes naturally is not always the nicest sort of thing. But when you're born again, you receive a new nature. And when you receive a new nature, then that's where the trouble always begins. Paul could say in the 7th of Romans that there was this battle going on between the old nature and the new nature. Paul could say, what I would not, I do it. That is, the new nature doesn't want to, but the old nature wants to do it. And the old nature is in control so that the time comes when you have to make a decision which one you're going to live by. You've got to make a determination, and that is in this matter of yielding to the Lord. You have to either permit the Holy Spirit to move in your life or else you have to do it by the energy of the flesh. There's no third alternative for the child of God. Now, you have something that's quite interesting here. The son of the bondwoman must be put out. That's exactly what we have here. And the thing was very grievous in Abraham's sight because of his son. After all, Ishmael's his son just as much as Isaac is, as far as the flesh is concerned. And after all, Isaac has just been born. A little bitty baby doesn't know too much about him yet, but this boy Ishmael has been in the home there, and Abraham's attached to him. And the thing's very grievous if he's going to have to send him away. Now, again, I go back to the thing we said at the time, that the thing that Sarah and Abraham did, God did not approve of it. Now, this is a heartbreak to Abraham. He's going to have to send this boy away. It's going to be a heartbreak to him. But God can't accept him. This sin is sin. God just didn't approve of it. And God doesn't intend to approve of it at all. It was a heartbreak to Abraham to send that boy away. And it was to relieve the embarrassment. And poor Sarah just couldn't take it with that boy around mocking her, this older boy. You can't live in both natures. You're either going to have to make a decision. James says a double-minded man's unstable in all of his ways. And that explains the instability, the insecurity among many Christians today. They want to go with the world, and yet they want to go with the Lord. They are spiritual schizophrenias. They are trying to do both, and you can't do that. They have a race among the Greeks years ago. They put two horses together, and a man puts one foot on one horse, one foot on the other, and he starts out. Well, that's a great race as long as the horses are together. But you and I have got two natures. One's a black horse, and the other's a white horse. And when they take out, I tell you, it's great if they go together. But these two horses, you just can't hitch them together. They won't work together. One goes one way and one the other. And when they do, you and I have to make up our mind which way we're going, whether we go in with one horse or with the other, with one nature or the other. And that's where we're told today to yield ourselves. Yield your members as instruments of righteousness. And what the flesh could not do, or the law could not do through the weakness of the flesh, the Spirit of God can now accomplish. The law tried to get a hold on man, but the old nature couldn't measure up to it. But now the Spirit of God can. That is the great message that you find here. Then we're going to see something else that took place here. That the child grew, and he was weaned, you'll notice. And Abraham then made a great feast because of that. And that was the thing that this little fellow did. He first lived by feeding on his mother's milk. But there came a day when he has to be weaned, and we're told today that newborn babes desire the sincere milk of the Word. You see a little baby in the crib, mama getting the bottle ready for him, and everything in his entire body is working. He's got his feet up in the air. He's got his hands up in the air, and he's yelling at the top of his voice. He wants his bottle. As newborn babes desire the sincere milk of the Word that you might grow, now there comes a day when you really want to grow up. And instead of just reading Psalm 23 and John 14, they are very wonderful. But instead of just reading those two chapters, then try going through the Bible. Read the entire Bible. Grow up. Don't be a babe all the time. So that we find this boy Isaac growing along, and he's a real lesson for us today. And that is something that you and I need to recognize. Now, this does begin now to reveal the character of the son of Hagar, Ishmael. This is the nature that you'll see manifested later on in the nation. A nation that is antagonistic. His hand is against his brother. And that's been the picture of him down through the centuries. That has been the picture of him. Now, there is something else for us to note here that is quite remarkable. And that is, in the birth of Isaac, you have, as I've already suggested, actually a comparison or a foreshadowing of the birth of the Lord Jesus Christ. And the birth of Isaac is that which sets it before us. And as we've said, God did not spring on mankind all of a sudden the virgin birth. He had had several miraculous births before this. The birth of John the Baptist was. The birth even of Samson was.