While the birth of Isaac filled Sarah's mouth with laughter, it introduced an entirely new element into Abraham's house. The son of the free woman very speedily brought about the development of the true character of the son of the bondswoman. Indeed, Isaac proved, in principle, to be to the household of Abraham what the implantation of the new nature is in the soul of a sinner. It was not Ishmael changed, but it was Isaac born. The son of the bondwoman could never be anything else but that. He might become a great nation, he might dwell in the wilderness and become an archer, he might become the father of twelve princes, but he was the son of the bondwoman all the while. On the contrary, no matter how weak and despised Isaac might be, he was the son of the free woman. His position and character, his standing and prospects, were all from the Lord. "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit." John 3:66That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. (John 3:6)....
The moment a sinner believes in his heart and confesses with his mouth, the Lord Jesus, he becomes the possessor of a new life, and that life is Christ. He is born of God, is a child of God, is a son of the free woman. (See Rom. 10:99That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. (Romans 10:9); Col. 3:44When Christ, who is our life, shall appear, then shall ye also appear with him in glory. (Colossians 3:4); 1 John 3:1, 21Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not. 2Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. (1 John 3:1‑2); Gal. 3:26; 4:3126For ye are all the children of God by faith in Christ Jesus. (Galatians 3:26)
31So then, brethren, we are not children of the bondwoman, but of the free. (Galatians 4:31).)
Nor does the introduction of this new nature alter in the slightest degree the true, essential character of the old. This latter continues what it was, and is made in no respect better; yea, rather, there is the full display of its evil character in opposition to the new element. "The flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other." There they are in all their distinctness and the one is only thrown into relief by the other.
I believe this doctrine of the two natures in the believer is not generally understood; and yet, so long as there is ignorance of it, the mind must be utterly at sea in reference to the true standing and privileges of the child of God. Some there are who think that regeneration is a certain change which the old nature undergoes and, moreover, that this change is general in its operation until, at length, the whole man becomes transformed. That this idea is unsound, can be proved by various quotations from the New Testament. For example, "The carnal mind is enmity against God." How can that which is thus spoken of ever undergo any improvement? The Apostle goes on to say. "It is not subject to the law of God, neither indeed can be." If it cannot be subject to the law of God, how can it be improved? How can it undergo any change? Again, "That which is born of the flesh is flesh." Do what you will with flesh, and it is flesh all the while. As Solomon says, "Though thou shouldest bray a fool in a mortar among wheat with a pestle, yet will not his foolishness depart from him." Pro. 27:2222Though thou shouldest bray a fool in a mortar among wheat with a pestle, yet will not his foolishness depart from him. (Proverbs 27:22). There is no use in seeking to make foolishness wise; you must introduce heavenly wisdom into the heart that has been heretofore only governed by folly. Again, "Ye have put off the old man" (Col. 3:99Lie not one to another, seeing that ye have put off the old man with his deeds; (Colossians 3:9)). He does not say, Ye have improved, or are seeking to improve "the old man," but, Ye have put it off. This gives a totally different idea. There is a very great difference between seeking to mend an old garment, and casting it aside altogether and putting on a new one. This is the idea of the last quoted passage. It is a putting off the old, and a putting on of the new. Nothing can be more distinct or simple....
The birth of Isaac did not improve Ishmael, but only brought out his real opposition to the child of the promise. He might have gone on very quietly and orderly till Isaac made his appearance; but then he showed what he was by persecuting and mocking at the child of resurrection. What then was the remedy?—to make Ishmael better? By no means; but, "Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac." Gen. 21:1010Wherefore 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. (Genesis 21:10). "That which is crooked cannot be made straight" (Eccles. 1:1515That which is crooked cannot be made straight: and that which is wanting cannot be numbered. (Ecclesiastes 1:15)); therefore you have only to get rid of the crooked thing altogether, and occupy yourself with that which is divinely straight. It is labor lost to seek to make a crooked thing straight. Hence all efforts after the improvement of nature are utterly futile so far as God is concerned. It may be all very well for men to cultivate and improve that which is of use to themselves; but God has given His children something infinitely better to do, even to cultivate that which is His own creation, the fruits of which, while they in no wise serve to exalt nature, are entirely to His praise and glory.