Now, 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 developed the true character of the son of the bond-woman. 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 bond-woman could never be any thing 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 bond-woman 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." Regeneration is not a change of the old nature, but the introduction of a new: it is the implantation of the nature or life of the second Adam, by the operation of the Holy Ghost, founded upon the accomplished redemption of Christ, and in full keeping with the sovereign will or counsel of God. 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. x. 9; Col. iii. 4; 1 John iii. 1, 2; Gal. iii. 26; iv. 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.