3 For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh,
4 that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit.
It was impossible for the law to 1) give freedom from condemnation, 2) justify, or 3) impart life by means of the flesh. The Law itself is perfect, but it couldn’t achieve its end because the flesh is weak (Romans 6:19).
sending His own Son — from glory to the world via the Incarnation. “Son” is stressed to show Christ’s power and greatness in contrast to the Law’s weakness — And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath He reconciled in the body of His flesh through death, to present you holy and unblameable and unreproveable in His sight (Colossians 1:21-22).
likeness — form. More than simple resemblance. He was in reality flesh, but without sin — identification and contrast.
and for sin = and as an offering for sin (Romans 3:25). A propitiation, not an atonement. (An atonement, or covering, describes Old Testament sacrifices.)
condemned sin — by the example of His life and by His crucifixion
righteousness of the Law — all it demands as right
fulfilled in us, not by us — It is the work of God in us, accomplished by the Holy Spirit.
walk — entire activity of a believer’s life
spirit — renewed inward man through whom the Holy Spirit operates.
1 Corinthians 1:30-31 — But of Him are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption: that, according as it is written, He that glorieth, let him glory in the Lord.
God doesn’t give us the power to live righteously, but does it in us and for us when we walk after the Spirit.