The cross is world-view shattering, according to Paul, for both Jews and Greeks (I Cor. 1:23). A crucified messiah could not be reconciled with a Jewish understanding that God does not work directly with the human world, but communicates indirectly through signs, through the law, through theophanies. God in the flesh contradicted the Jewish understanding that no one has seen God directly, not even Moses, who may have seen a residue of his glory. God might make an appearance in fire and cloud and burning bush, but it was understood that these were signs of God and not God himself. On the other hand, Greek wisdom has the opposite problem, in that to attain to final and full wisdom will mean a passing beyond the physical world to the forms, to the logos, to the abstract and propositional. For Plato, the body is the prison of the soul, and the physical world is a realm of shadows which only true wisdom can escape. The notion that wisdom could be embodied in a single, incarnate individual contradicts the Greek world-view. While wisdom might be obtained by the peculiarly wise or devout, the very nature of wisdom was disincarnate and impersonal.
In both the Jewish and Greek sense then, the cross as a bringing together of the most human of events (death, dying and suffering) with the divine would require a relinquishing of their view of the world and of God. There is no continuity or natural progression from Judaism and Greek notions of wisdom to the cross; the cross requires a shift in world-views. For Jews the cross is a sign that bears too much weight as God is upon it, and for the Greeks the cross is a foolish impossibility as wisdom transports one beyond the body and beyond the physical.
Paul expresses the Jewish attachment to signs as a preference for the letter of the law more than what the letter signifies. For Jews there was an infinite gap between the reality of God and the reality of the world and the best one could do was to cling to the letter, to the sign, to the appearance, and Paul considered this deadly. It is not that Paul considered writing or language as inherently deficient, as if it is the sign system which is inadequate for making God known. The letter kills, in his estimate, not because of some inherent quality in letters, writing, and signs.
Writing and signs as an end in and of themselves are on the order of flesh and blood as an end in and of themselves. It is not that Paul thinks that flesh and blood are bad or inadequate, it is just that flesh and blood and signs cannot be taken as “sufficient.” In fact, the right combination of flesh and writing – writing not on tablets of stone but writing on the flesh of the heart – he equates with an encounter with “the Spirit of the living God” (2 Cor. 3:3). The problem is not with writing or signs per se, but with what is used to write and what is written upon. In Paul’s description of the ideal, the flesh of the heart is the writing tablet and the Spirit of God is the writing instrument. Paul does not denigrate the flesh, as if one has to rid themselves of embodiment and flesh so as to receive the Spirit. It is precisely the flesh of the heart that receives the imprint of the Spirit. (In Galatians and Romans, he will use the term “flesh” as shorthand for the danger he is touching upon in Corinthians, but as I explain here, this is in no way a flesh/Spirit dualism.)
The Gnostics, who seem to combine Greek wisdom with a Jewish understanding, will take up a small portion of Paul’s argument in Corinthians (particularly I Cor. 15:53-54) to argue that the way one passes into spirituality is to relinquish physicality. Paul is arguing that one puts on immortality, not through a subtraction of physicality, but through an addition of spirituality. Christ does not pass beyond physicality to glory, as in the Gnostic interpretation, but his most fleshly, physical moment in his suffering on the cross, is the moment of his glory. Immortality, incorruption, and spirituality do not involve disembodiment but a writing upon the flesh with the Spirit. Specifically, what gets written on the flesh is the gospel of Christ, focused as it is upon the cross.
The Gnostic inclination, which is the universal inclination (both Jew and Greek), stumbles over the physical, the embodied, the created world. The letter trips them up in an all or nothing sort of way, so that it is presumed one either clings to the letter, the body, physicality, or one clings to the Spirit. This may seem to be a matter of semantics, and may be in certain instances, but Paul’s point is that the letter or the flesh isolated from the Spirit kills. The isolation may occur because one imagines the law or the letter is all there is, or the isolation may occur because the letter, the physical sign, and all things physical are considered inherently inadequate. In both instances the letter is divided from the Spirit, and this division is deadly.
The deadliness, in Paul’s explanation concerns placing “sufficiency” in the wrong place. One might presume the law is “adequate” or that “we are sufficient of ourselves” (2 Cor. 3:5) which seems to amount to the same thing. The difference is that “we ourselves” may attain sufficiency but this is because “our sufficiency is from God.” He is the one who “made us sufficient as ministers of the new covenant” (3:6). There is no need to get rid of ourselves or a portion of what constitutes us as selves, namely the body, the heart, or human physicality.
On the other hand, clinging to the letter of the law amounts to positing sufficiency in “the ministry of death” in which what is written is permanently removed from the flesh of the heart, because it is “engraved on stones” (3:7). Paul describes it as containing an inherent deception in which the fading glory or the ultimate inadequacy of the law is veiled in the same way Moses face was veiled. Their “minds were blinded” and continue to be blinded to the fading glory “of what was passing away” (3:12). It is not that writing, physicality, bodies, or faces are inherently corrupt, it is a matter of removing the blinding veil in which the law is taken to be an end in and of itself. Paul assures his readers, “the veil is taken away in Christ” (3:14) and with its removal the Spirit does its work on the body: “But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord” (3:18).
As Paul explains in the next chapter, it is not because “we have this treasure in earthen vessels” (4:7) that causes the problem. In fact, the very point is precisely that the earthen vessels accentuate the power of God: “But we have this treasure in earthen vessels, that the excellence of the power may be of God and not of us” (4:7). The human body, in its physicality and mortality, allows for taking up the fulness of the image of Christ. Paul describes himself as “always carrying about in the body the dying of the Lord Jesus, that the life of Jesus also may be manifested in our body” (4:10). The reality of life in God is realized through a continual engagement with mortal flesh: “For we who live are always delivered to death for Jesus’ sake, that the life of Jesus also may be manifested in our mortal flesh” (4:11). The flesh and body are not extrinsic to salvation but the very sight of redemption and transformation showing forth the life of God.
 He equates the body with a prison for the soul in Phaedo: “‘I will tell you,’ he replied. ‘The lovers of knowledge,’ said he, ‘perceive that when philosophy first takes possession of their soul it is entirely fastened and welded to the body and is compelled to regard realities through the body as through prison bars, not with its own unhindered vision, and is wallowing in utter ignorance.’” Plato in Twelve Volumes, Vol. 1 translated by Harold North Fowler; Introduction by W.R.M. Lamb (Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1966) 83.
 This is the argument Irenaeus uses against the Gnostics who have latched on to this verse in Corinthians to prove that one must get rid of flesh and blood to enter the Kingdom: “As, therefore, the bride cannot [be said] to wed, but to be wedded, when the bridegroom comes and takes her, so also the flesh cannot by itself possess the kingdom of God by inheritance; but it can be taken for an inheritance into the kingdom of God.” Against Heresies (Book V, 9.4).