Transcribed by Jeff Fenske from “Life in the Son: A Study of the Doctrine of Perseverance,” by Robert Shank, 1960, 1961 and 1989, p. 212-215.

A popular doctrine … is the assertion that the believer has two natures, one of which is carnal and can do nothing but sin, and the other of which is spiritual and cannot possibly sin. It is assumed that, when a Christian sins, it is only a manifestation of his old carnal nature; but his new spiritual nature is not involved. …
The doctrine of the two natures of the believer” is an erroneous assumption. It is true that the believer’s own inherent nature is carnal, and certainly it is the occasion of sin—to his sorrow [6]. But contrary to the assumptions of some, the believer does not possess two natures as equally as his own, for he does not possess the new spiritual nature as his own, per se. Rather, he is only a “partaker of the divine nature” (II Pet. 1:4), “who partakes of His holiness” (Heb. 12:10) through submission and faith in Jesus Christ. …
Despite the assumptions of many, the Christian does not have two natures. To the contrary, he is a single spiritual entity who can act only as an integer [def.: “a whole unit or entity” – jeff]. … The whole of his person is involved in whatever he does “after the Spirit,” and in whatever he does “after the flesh.” As a spiritual integer, he is continually confronted with the necessity of choosing whether to walk after the flesh, or after the Spirit; whether to sow to the flesh, the ultimate outcome of which is to reap destruction, or to sow to the Spirit, the consequence of which is to reap life everlasting. Whichever he does he can do only as the whole man.
… And men can walk but one direction at a time. Whether a man walks “after the Spirit” or “after the flesh,” he walks with both feet as the whole man.
The popular concept of “the strife of the two natures of the believer” may seem to be substantiated by Paul’s account of the conflict which he experienced within himself, as recorded in Romans 7:7-25. But the passage, which is historical and autobiographical, depicts the conflict between conscience and conduct, between aspirations and inclinations, which Paul experienced while still under the Law. (The “therefore” of 8:1 reverts to 7:6, and 7:7-25 is parenthetic.) The dismal conclusion which Paul reached in the conflict within himself under Law is stated (following a parenthetic exclamation in vv. 24, 25a) in verse 25b: “So then with the mind I myself serve the law of God; but with the flesh the law of sin.” But now that he has found deliverance through Jesus Christ our Lord and by faith has come to be “in Christ Jesus” (8:1), no longer can he resign himself to such a miserable conclusion. Far from accepting such a conclusion, Paul insists that we who are in Christ Jesus must “walk, not after the flesh, but after the Spirit” (v. 4). For the man who is “in Christ Jesus,” such a course is mandatory. But it is not inevitable, even though he is “not in the flesh but in the Spirit” through His indwelling presence (v. 9). The Christian must continue to choose whether to be “carnally minded,“ which tends toward death, or to be “spiritually minded,” which tends toward life and peace (v. 6). Paul sharply admonishes his brethren who are in the Spirit to remember that “we are debtors, not to the flesh, to live after the flesh” (v. 12). We must recognize that two courses are open to us, and we can follow but one. We must weigh carefully the consequences of following either course. If we live after the flesh, we shall die; if we live after the Spirit, we shall live (v. 13). Only men who consent to be led of the Spirit can be sons of God (v. 14), and only such as consent to suffer with Christ will ultimately be glorified together with Him (v. 17).

Related:
THE PATRISTIC INTERPRETATION of ROMANS 7:14-25 — The early church did not understand Rom 7:14-25 to teach the necessity of sin in believers
Leonard Ravenhill: Romans 7 vs 8