Romans 8:3

Again, he seems indeed to be disparaging the Law. But if any one attends strictly, he even highly praises it, by showing that it harmonizes with Christ, and gives preference to the same things. For he does not speak of the badness of the Law, but of "what it could not do;" and so again, "in that it was weak," not, "in that it was mischievous, or designing." And even weakness he does not ascribe to it, but to the flesh, as he says, "in that it was weak through the flesh," using the word "flesh" here again not for the essence and subsistency itself, but giving its name to the more carnal sort of mind. In which way he acquits both the body and the Law of any accusation. Yet not in this way only, but by what comes next also. For supposing the Law to be of the contrary part, how was it Christ came to its assistance, and fulfilled its requisitions, and lent it a helping hand by condemning sin in the flesh? For this was what was lacking, since in the soul the Lord had condemned it long ago. What then? is it the greater thing that the Law accomplished, but the less that the Only-Begotten did? Surely not. For it was God that was the principal doer of that also, in that He gave us the law of nature, and added the written one to it. Again, there were no use of the greater, if the lesser had not been supplied. For what good is it to know what things ought to be done, if a man does not follow it out? None, for it were but a greater condemnation. And so He that hath saved the soul it is, Who hath made the flesh also easy to bridle. For to teach is easy, but to show besides a way in which these things were easily done, this is the marvel. Now it was for this that the Only-Begotten came, and did not depart before He had set us free from this difficulty. But what is greater, is the method of the victory; for He took none other flesh, but this very one which was beset with troubles. So it is as if any one were to see in the street a vile woman of the baser sort being beaten, and were to say he was her son, when he was the king's, and so to get her free from those who ill treated her. And this He really did, in that He confessed that He was the Son of Man, and stood by it (i.e. the flesh), and condemned the sin. However, He did not endure to smite it besides; or rather, He smote it with the blow of His death, but in this very act it was not the smitten flesh which was condemned and perished, but the sin which had been smiting.

The reason why grace was bestowed on us through our mediator is that we who were polluted by sinful flesh might be purified by the "likeness of sinful flesh."