St. Paul talks about Flesh vs. Spirit. By flesh he does not mean "the body". We know this because the "works of the flesh" that he lists include spiritual as well as corporal sins. By flesh (sarx
in Greek) he means the inborn tendency of the Old Adam to follow his own selfish will straight into sin, corruption and death. The Flesh is thus that part of us that ignores God and sets ourselves up as god in his place; it is the unredeemed man, it is man steeped in original and actual sin.
This is contrasted with the Spirit, which is both the Holy Spirit (in a specific sense) and also (in a more general sense) the New Creation - the life of God in us. Paul says he was "crucified with Christ" - "and it is no longer I who live but Christ who lives in me." The fruit of the Spirit is sanctification and everlasting life. It is a new thing, new wine poured into new wineskins. It is the re-creation of our whole selves, body and soul.
But to turn toward the new life of God, we must mortify or "put to death" the old life in us.
For if you live according to the flesh you will die, but if by the Spirit you put to death the works of the flesh, you will live. (Rom. 8:13)
The "flesh" is that desire in us that seeks to fill that empty hole, that horrible little hole, that thing that is easily abused and threatened and that eventually will die - that thing that wants more and more and is never satisfied. It's the Hungry I, the petty tyrant in each of us.
And Paul says that one of the works of the flesh is "divisions" or "factions". Know, then, that when you see disunity, the root cause of it, on one side or the other, is always an agenda, a selfish program, a self-serving desire to get one's own way. In theological disputes, one side is usually pushing just such an agenda, a reading of Scripture and Church teaching and the Catechism that allows for a specific work of the flesh, a particular sin that is "preciousss" to the sinner.
Spot the agenda, see how the dissenter is "fleshing it out", and understand how the disunity takes root, sprouts and grows.
This is true when people turn somersaults to rationalize Lying, when they bend over backwards to make excuses for bad theologies about sex, and even when they argue about Sacred Music.
And how can you tell which side has a fleshy agenda and which side doesn't? When the argument stops being about discovering truth and becomes irrational or personal, then a nerve has been touched.
When people argue in Bad Faith, it's not an argument in the Spirit; it's an argument in the flesh.