From Luke Timothy Johnson’s Reading Romans (2001), 48:

Paul’s starting point is the analysis of idolatry in Romans 1:18–32. Jews thought of idolatry as a matter of worshiping the wrong gods, and therefore something that only Gentiles could do. Paul thought more deeply on the matter. He saw that idolatry was a disease of human freedom, found as widely among Jews as among Gentiles.

Idolatry begins where faith begins, in the perception of human existence as contingent and needy. But whereas faith accepts such contingency as also a gift from a loving creator from whom both existence and worth derive, idolatry refuses a dependent relationship on God. It seeks to establish one’s own existence and worth apart from the claim of God by effort and striving (“works”) of one’s own.

Paul will use the striking expression “the flesh” (sarx) and speak of “life according to the flesh” (Romans 7:5, 18, 25; 8:3–7). He means by flesh the measurement of life apart from spirit, and specifically apart from the Holy Spirit of God. It is life in denial of transcendence, a life lived on the basis of perceived reality, taken as a closed system. Seeking to establish one’s own life and worth within such a framework requires boasting and arrogance. It demands competition and hostility toward others.

The reason is simple. Since life as a gift is rejected, then life on one’s own terms must be by means of having or possessing. I am insofar as I have, own, can claim, “this is mine.” And since I view the world as a closed system, there is only so much “having” available. I am inevitably in competition with other humans for life and worth. My self-aggrandizement must be at another’s expense. Rivalry, envy, hatred, and murder are the logical expressions of the idolatrous impulse, for the “need to be” that derives from the refusal of the first gift is an endless hunger, an unslakable thirst.