In the middle of Romans, Paul describes Jesus' incarnation and atoning sacrifice:
8:1 Therefore there is now no condemnation for those who are in Christ Jesus.
2 For the law of the Spirit of life in Christ Jesus has set you free from the law of sin and of death.
3 For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh,
4 so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit.
The phrase "likeness of sinful flesh" has caused consternation for almost two thousand years; ever since the heretic Marcion seized on it to claim that Jesus had not come as a human, but as a divine being with a different physical nature. Jesus only appeared to be human according to Marcion, whose docetic tendencies found the idea of a flesh and blood human Christ-figure as repulsive. To docetists like Marcion, the material world was evil and therefore Christ would not have partaken in it and he therefore was made of some other kind of stuff.
Marcion's interpretation of this passage met with a forceful response from Tertullian, a Church Father writing in the early 200s:
Now in another sentence he says that Christ was "in the likeness of sinful flesh," not, however, as if He had taken on Him "the likeness of the flesh," in the sense of a semblance of body instead of its reality; but he means us to understand likeness to the flesh which sinned, because the flesh of Christ, which committed no sin itself, resembled that which had sinned,--resembled it in its nature, but not in the corruption it received from Adam; whence we also affirm that there was in Christ the same flesh as that whose nature in man is sinful. In the flesh, therefore, we say that sin has been abolished, because in Christ that same flesh is maintained without sin, which in than was not maintained without sin.
Is Tertullian's take on the verse superior to Marcion's?
Very much so.
Paul is elsewhere quite clear that Jesus was a human being, "according to the flesh." In my article on the phrase "according to the flesh," I explore several such verses. Most prominent are Romans 1:3-4 where Paul says Jesus was born a descendent of David "according to the flesh" and speaks of literal descent from Abraham "according to the flesh." Colossians states that Jesus reconciled us "in His fleshly body." 1:22. So if Paul affirms a Jesus in the flesh elsewhere, why does he backtrack here and limit himself Jesus to simply being in the "likeness"?
Tertullian puts us on the right path. Paul is not talking in 8:3 about Jesus being made of the same stuff ("flesh") as human beings. He does not say that Jesus came in the "likeness of flesh" but in the "likeness of sinful flesh." This is confirmed by Paul himself in the second part of the same verse where he states, "He condemned sin in the flesh." Jesus defeated sin by becoming a human being in the flesh. What he did not do is become sinful flesh. He never succumbed to the binding power of sin that controls sinful man. As F.F. Bruce states:
The humanity of Christ is shared by him with all mankind. But ours is “sinful flesh”, because sin has established a bridgehead in our life by means of which it dominates the human situation. Christ came in real flesh – he lived and died in a “body of flesh” (Colossians 1:22) – but he did not come in “sinful flesh”, because sin gained no foothold in his life; he is said therefore to have come “in the likeness of sinful flesh,” so that, when he presented his life as a sin-offering, God thus “condemned sin in the flesh” (Romans 8:3) – passed the death-sentence on it by virtue of the sinless humanity of Christ.
F.F. Bruce, Paul, Apostle of the Heart Set Free, page 205.
Also see Ben Witherington's explanation:
The juxtaposition of Rom. 8:3 with 8:4 makes it very likely that Paul is implying that like Adam, Jesus was born with an unfallen nature, though one that had the capacity to sin, but unlike Adam, Jesus remained sinless and so could be an unblemished sacrifice for sin. Thus Paul uses the term “likeness of sinful flesh" not to deny Jesus’ sinlessness in regard to his behavior (cf. 2 Cor. 5:21), nor to deny his sinless human nature, but to affirm that he was in the flesh and was in this respect like all other human beings, “save without sin” (cf. Heb. 4:15, 2:17). The eschatological Adam starts humanity over again without blemish. A further confirmation of this interpretation can be found in Phil. 2:7-8, where we are told Christ is born in the likeness of human beings. This is intended to tell us that Jesus took on human flesh, being a man who was subject to human frailty and weakness, even death. Romans 8:3 is not suggesting anything significantly different from Phil. 2:7-8.
Ben Witherington, Paul’s Narrative Thought World, page 140.
Witherington's reference to Phil. 2:7-8 allows us to kill two birds with one stone. Verses 7-8 state that Jesus "emptied Himself, taking the form of a bond-servant, and being made in the likeness of men. Being found in appearance as a man..." Here, the terms "likeness" and "appearance" are not intended to deny that Jesus came in the flesh. Jesus became flesh, but did not partake of sinful human existence. He did not cease being divine. As Gordon D. Fee notes, these terms are,
used primarily because of Paul's belief (in common with the rest of the early church) that in becoming human Christ did not thereby cease to be divine. This word allows for the ambiguity, emphasizing that he is similar to our humanity in some respects and dissimilar in others. The similarity lies with his full humanity; in his incarnation he was "like" in the sense of "the same as." The dissimilarity, which in Rom 8:3 had to do with his being sinless while in the "likeness" of sinful flesh, in this case has to do with his never ceasing to be "equal with God." Thus he came in the "likeness" of human beings, because on the one hand he has fully identified with us, and because on the other hand in becoming human he was not "human" only. He was God living out a truly human life, all of which is safeguarded by this expression.
Fee, Paul's Letter to the Philippians, pages 213-24.
I think Fee touches on some of the modern confusion over this passage. We should remember that Paul was not expressing refined theological treatises. He was trying to explain something that he himself may not have fully understood. Jesus become human, but was also divine. He became flesh, but not fleshly. He had to become a human to reconcile man to God, but had to be a perfect, sinless sacrifice to accomplish that end. In other words, it was important to Paul's theology that Jesus be human, but also important that Jesus not share in humanity's sinfulness (which, afterall, was a key part of the existence of humans). It was important that he recognize Jesus' preexistence and divinity, but not at the expense of his becoming a human being, which he had to be in order to accomplish His purpose. It is these considerations that lead Paul to choose his words carefully when saying that Jesus took on only the "likeness" of sinful flesh and that he took on the "appearance" of man.
When read in light of Paul's other affirmations of Jesus becoming flesh, his statement within the same verse that Jesus condemned sin "in the flesh", and recognizing the fine line between the theological concepts Paul was juggling, it is clear that Marcion was wrong and Tertullian correct. Paul believed in a Jesus who was made of flesh like the rest of humanity, but also in a Jesus who did not share or succumb to humanity's sinful nature.