Romans 8:3 says, "For that which the law could not do, in that it was weak through the flesh...." This represents the true condition of the morally minded who constitute a portion of fleshy men. They may perhaps be willing enough to devote themselves to the keeping of the law, but they are fleshy. Being weak, they are incapable of keeping the whole law. These people come under one category. There is yet another category of men who do not keep the law of God at all. Their mind is the "mind set on the flesh" which is "enmity against God; for it is not subject to the law of God, for neither can it be" (v. 7). However, the law decrees that he who keeps the law lives by the law, and he who does not keep it shall be condemned unto perdition. How much of the law must then be kept? The answer is the whole law, for "whoever keeps the whole law yet stumbles in one point has become guilty of all" (James 2:10). "Because out of the works of the law no flesh shall be justified before Him; for through the law is the clear knowledge of sin" (Rom. 3:20). So the more man strives to keep the law, the more conscious he is of his being full of sins, incapable of keeping the law of God. Therefore, the first part of Romans 8:3 points out to us the condition of man and how sinful he is.
Since all men are sinful, God prepared to save them, and His way of salvation was by "sending His own Son in the likeness of the flesh of sin." His Son is without sin, so He alone is able to save. "In the likeness of the flesh of sin" refers to the birth of the Lord Jesus on earth, in which He took on a human body and identified Himself with humanity. God’s own Son as the "Word" came "in the likeness of the flesh of sin," that is, He became flesh. This verse, therefore, speaks of incarnation. The important point contained herein is that He is the Son of God and He is without sin. The text does not say He was made "sinful flesh" but rather "in the likeness of the flesh of sin;" that is to say, He became flesh, having the likeness of the sinful body of man. Although He became flesh, He is still the Son of God and thus is still without sin. But as He also has the likeness of the sinful flesh of man, He is very intimately affiliated with sinners of the flesh in the world.
What was the purpose of the Lord’s incarnation? It was to be made "a sacrifice for sin" (Rom. 8:3, Darby, margin); this is the work of the cross. God’s Son is for the redemption of sin. Those who are fleshy sin against the law and cannot fulfill the righteousness of God, so they must perish and suffer the punishment for sin. However, the Lord came to the world, took on the likeness of the flesh of sin, and wholly identified Himself with all fleshly men. So, when He died on the cross, all fleshy men were judged to have received punishment for their sins in Him. He is sinless and need not have suffered punishment, but when He received the punishment, He had the likeness of sinful flesh. Accordingly, as the Head of a new race, He included all sinners in Him in suffering the punishment. This is insofar as punishment is concerned.
The fleshly men who were to receive the punishment now have in Him a sacrifice for sin. But what is to be done about the flesh that is filled with sins? He "condemned sin in the flesh." It was for sin that He died; thus God made the Sinless become sin on our behalf. When the Lord Jesus died, He died in the flesh, "being put to death in the flesh" (1 Pet. 3:18). When He died in the flesh, the sins borne by His flesh were crucified with Him. This is what is meant by "condemned sin in the flesh." In other words, it is "to judge sin in the flesh" or "to convict sin in the flesh." To condemn means to judge or to punish. The judgment and punishment for sin is death. What is meant here is that He put sin to death in the flesh. It can therefore be seen that when the Lord Jesus died, He not only died as a sacrifice for sin, suffering the punishment on behalf of men, but He also brought sin under punishment. As sin was condemned in the Lord’s body at the time of His death, so is sin condemned in the flesh of him who is joined to the Lord’s death; sin no longer has the power to harm him.
(Spiritual Man, The (3 volume set), Chapter 5, by Watchman Nee)