How do we understand the concept of “living after the flesh”?

St. Augustine says, “That we might not fall from continence, we ought to be especially vigilant against the treachery of diabolical suggestions to presume in our own strength. ‘For cursed be the one that trusts in man’ (Jer. 17:5). And who is that one but man? For, he cannot truthfully say that he does not put his trust in man who puts it in himself. And this, also, to live according to man, what is it except to live according to the flesh? Let him who is tempted by such a suggestion therefore, listen, and if there is in him any Christian feeling, let him

815 St. Augustine of Hippo, On Continence 3.7 (FOTC 16:196) amended. Many people fall into this deception. For example, when a young man falls in love with a young lady and becomes emotionally attached to her, he may find that the temptation to gaze lustfully no longer assails him, nor do impure thoughts preoccupy him. Finding himself in this state, he considers himself to have become saintly, not realizing that the lusts within him are merely resting from their corruption since there is currently no need for them to rise up and rebel within him because they are in fact effectively possessing and ruling him under the guise of what looks like holiness. The opposite also occurs when a person breaks off a relationship. The war of thoughts rekindles, and lustful gazes towards her and towards others increase, for the lust of the flesh opposes the desire of the spirit to be chaste.

816 St. Augustine of Hippo, On Continence 3.8 (FOTC 16:197–198).

tremble; let him hear this, I say: ‘If you live according to the flesh, you shall die’ (Rom. 8:13).”⁸¹⁷