We must be careful not to adopt the overly simplistic philosophical ideology of “flesh” being universally bad and “spirit” as being universally good. Gnostic Dualism is the technical name of this notion. Basically, this concerns a person being divided between two natures – flesh (bad) and spirit (good). Plato instituted this philosophy long before Christianity, Gnostics adapted it to Christianity, and Christians today have accepted the thought as biblical and good practice.
This dualistic understanding is a basis for the doctrine that, since the flesh is bad, anything that pleases the flesh is bad as well. Likewise, since the spirit is good, anything that pleases the spirit must be good, too. Thus, specifically in medieval times, we had the onset of Asceticism (a complete separation from the world and worldly things, and a complete focus on the spirit and spiritual things).
The issues with this thinking are, initially, that it is based on a fundamental flaw. God made humanity of flesh and, therefore, it cannot be bad. Secondly, Christ “became flesh” and lived among us (John 1:14). He had a different nature, but the same flesh as we do (Rom. 8:3), was perfect and, therefore, not bad.
The confusion is founded (once again) in a miss-translation of the Pauline letters. First, we often mistakenly believe that Paul spoke English and that his writings and words carry with them English definitions. Second, the question of what Paul means when he uses the word sarx (“flesh”)? And thirdly, Paul draws a distinction between “flesh” and “Spirit” (with a capital “S”), which leads to something completely different in the human “spirit” (with a lowercase “s”).
Sarx, in Paul’s usage, has at least three (and as many as six) different meanings. Sometimes he uses it to speak generally of physical matter (1Cor. 15:39). Other times, he is speaking of the human body itself (1Cor. 6:16). Then again, the human race in particular (Gal. 2:16; 1Cor. 1:29) is in view. Now, he also uses it to speak of Christ’s human lineage over against His divine existence (1 Tim. 3:16; Phil. 2:9-11). He also speaks of the value system of fallen humanity (1Cor. 1:26; 2Cor. 10:3-4; Phil. 3:3-4). But his most characteristically common use of sarx is as that which stands in opposition to God (Rom. 5:12-21; Chapter 8; Gal. 3:2-3; 5:22-26).
The point for Christians (and the Church) is that the flesh, because of the fallen nature of humanity as a whole, seeks its own (and not God). The guilt of sin and the punishment of death are housed in the flesh. The flesh can never please God and it can never do enough “good works” to change that fact. The new nature has nothing to do with the flesh and everything to do with the Spirit. It is the Spirit of Christ which mingles with the human spirit which brings about a “new creation.” It is not because it is “spirit” that it is good, but because it is the work of the Holy “Spirit” in us for good (Rom. 8:1-11). Faith has nothing to do with the flesh. Doing the best we can has nothing to do with faith, but entirely flesh. And the flesh has been crucified with Christ (Rom. 6:6; Gal. 2:20).
Incidentally, if Christianity were about the flesh, then why is there a resurrection of the dead (1Cor. Chapter 15)?