Matthew Poole and continuators on Romans 8:4

4 That the righteousness of the Law might be fulfilled in us , who walk not after the flesh, but after the Spirit .

Here is another end of Gods sending his Son as before, it was that he might perfectly fulfil the Righteousness of the Law in or for us, which for us our selves to do in our own Persons was utterly impossible. And yet upon which (as being imputed unto them, and accepted of God on our behalf) we shall be accounted Just and Righteous, as if we had done it our selves. Christs being a Sacrifice for sin was not sufficient to answer all the ends and demands of the Law; there must be doing of what it commanded, as well as suffering of what it threatned: Therefore Christ was sent for both, and both were accomplished by him; and what he did and suffered is accounted unto us as if we had done and suffered it. This is the imputed Righteousness which was so often spoken of Chap. 4. and in reference to this he is said to be made Righteousness for us, 1 Cor. 1.30. and we are said to be the Righteousness of God in him, chap. 5.19. 2 Cor. 5.21.

This was the description before of those that had Union with Christ, and exemption from Condemnation; and 'tis again set down as the description of those who partake of the Righteousness of Christ in this way of imputation; and 'tis added here again, to stave off all others from laying claim to this grace. None but holy walkers can warrantably apply Christs fulfilling or satisfying the Law to themselves, because Christ hath fulfilled the righteousness of the Law for us; none may infer there is nothing for us to do; we may live as we list: for though Christ hath fulfilled the Law in all respects, yet all those for whom he hath so done, or have benefit thereby, are, and must be such as walk not after the flesh, but after the spirit: for the opening of which terms, vid. v. 1.