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 gradual 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." (Prov. xxvii. 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. iii. 9.) He does not say, Ye have improved or are seeking to improve "the old man;" but, Ye have put it off. This gives us 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.
Passages might easily be multiplied to prove the unsoundness of the theory, with respect to the gradual improvement of the old nature,--to prove that the old nature is dead in sins, and utterly unrenewable and unimproveable; and, moreover, that the only thing we can do with it is, to keep it under our feet in the power of that new life which we have in union with our risen Head in the heavens.
The birth of Isaac did not improve Ishmael, but only brought out his real opposition to the child of 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 bond-woman and her son; for the son of this bond-woman shall not be heir with my son, even with Isaac." (8-10.) Here was the only remedy. "That which is crooked cannot be made straight;" 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.