Sarx literally means flesh, but in Paul's letters it takes on a deeper sense: the fallen, self-centred human nature, prone to sin and set against God. It is not the body itself that is evil — God made the body good — but sarx names the old nature that pulls us toward selfishness and away from the Spirit.
The Christian life involves a real conflict: the flesh lusteth against the Spirit, and the Spirit against the flesh. Victory does not come by white-knuckled willpower but by walking in the Spirit, so that we do not fulfil the desires of the flesh. To grasp sarx is to understand why the believer is not yet fully free of struggle — and why the answer is always to lean harder into the Spirit, not into ourselves.
Sarx in Scripture
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.