The following is a portion of the Gospel reading for this past Sunday (21st ordinary) at Mass:
Many of Jesus’ disciples who were listening said,
“This saying is hard; who can accept it?”
Since Jesus knew that his disciples were murmuring about this,
he said to them, “Does this shock you?
What if you were to see the Son of Man ascending
to where he was before?
It is the spirit that gives life,
while the flesh is of no avail.
The words I have spoken to you are Spirit and life.
But there are some of you who do not believe.”
That about which our Lord’s disciples were murmuring was, of course, His teaching on the mystery of the Eucharist –– His flesh and blood, His very life, given to us by the Father in the Spirit. A common tactic used by critics of the Catholic teaching on this mystery (transubstantiation, Real Presence, etc.), is to note how Jesus negates the relevance of “flesh” to His teaching about eternal life in Him: “It is the spirit that gives life, while the flesh is of no avail” (6:63). This is, however, a laughable tactic, since the better half of chapter 6 is about the absolute necessity of consuming Christ’s flesh for salvation (6:51, 6:53–56). It is crucial to note that in all His most explicitly eucharistic verses, Christ speaks of “my flesh”, or “the flesh of the Son of Man”, and only after half a dozen such usages does He contrast “my flesh” with “the flesh”. The flesh, as a would-be principle of autonomy –– the world, in other words, devoid of Christ’s redemptive immersion in it (Incarnation) –– the flesh is presented as the antithesis of “the Spirit”, which I am thinking of here as the principle of infinite vitality, creative power, and truth (John 3:5, 14:17, 15:26).
Five chapters earlier the link between the Eucharist and the Incarnation –– that is, the fundamental link between the sacrament and salvation –– had been made in these words (1:1, 1:14–18):
In the beginning was the Word, and the Word was with God, and the Word was God. … He was in the world, and the world was made through him, yet the world knew him not. He came to his own home, and his own people received him not. But to all who received him, who believed in his name, he gave power to become children of God;  who were born, not of blood nor of the will of the flesh (θελήματος σαρκὸς) nor of the will of man (θελήματος ἀνδρὸς), but of God.  And the Word became flesh (ὁ λόγος σὰρξ ἐγένετο) and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father.
Notice again the subtle but crucial difference between “flesh” (σὰρξ) and “the flesh” (ἡ σὰρξ), as indicated both in John 1 (v. 14) and in John 6 (v. 63). Flesh per se is not an obstacle to God; indeed, it has become the means by which all creation is redeemed to the Father in Christ. When flesh, however, becomes an end in itself, becomes “the flesh” pro se, it “is of no avail”. The saving bridge which brings flesh from it doomed insularity is nothing less than that which Christ presents to us as the bread of life (ὁ ἄρτος τῆς ζωῆς, 6:35), “my flesh” (μου τὴν σάρκα, 6:54).
Since I am not by any means a scholar of Greek (much less else!), I will conclude by saying that the lack of “the” in John 1:13 is surely due to a genitive transformation and not a theological confusion. John 8:15 speaks of “the flesh” (τὴν σάρκα) as a faulty standard of wisdom. In John 17:2 Christ speaks of His power over “all flesh” (πάσης σαρκός). These verses (which seem to be the last instances of “flesh” in the Gospel of John) add further support and insight to the distinction between “the flesh”, “flesh”, and “my flesh”. Additionally, John 3:15 seems to give a special support to my point: “That which is born of the flesh (τῆς σαρκὸς) is flesh, and that which is born of the Spirit (τοῦ πνεύματος) is spirit.” To wit, “the flesh” produces ‘mere’ flesh and finitude, while “the Spirit” (cf. John 14:26) produces spirit and life.
Just something to keep in mind at your next Communion. Indeed, much of this post came to me during the Offertory before communion this past Sunday.