Paul F Pavao
Psalm 1 is short, so the early Christian commentary is sorted by date rather than by verse.
Since man is a living being compounded of soul and flesh, he must exist by both of these. And, since offenses come from both of them, purity of the flesh is restraining, in abstinence, from all shameful things and all unrighteous deeds. Purity of the soul is keeping faith towards God complete, neither adding to it nor diminishing from it.
Godliness is obscured and dulled by the soiling and staining of the flesh. It is broken, polluted, and no longer complete, if falsehood enters into the soul, but it will keep itself in its beauty and not diminish when truth is constant in the soul and purity in the flesh.
For of what benefit is it to know the truth in words, but to pollute the flesh and perform the works of evil? Or, what benefit can purity of the flesh bring, if truth is not in the soul? These rejoice with one another and are united and allied to bring man face to face with God.
Therefore the Holy Spirit says by David, "Blessed is the man who has not walked in the counsel of the ungodly," that is, the counsel of the nations who do not know God, for the ones who are ungodly are those who do not worship the God that truly is. Therefore the Word says to Moses, "I AM HE THAT IS," but those who do not worship the God that is, these are ungodly.
"And has not stood in the way of sinners, but sinners are those who do not know God and do not keep his commandments; that is, disdainful scorners.
"And has not sat in the seat of the pestilent." Now the pestilent are those w ho by wicked and perverse doctrines corrupt not only themselves, but others also, For the seat is a symbol of teaching. Such then are all heretics. They sit in the seats of the pestilent, and the ones who are corrupted are those who receive the venom of their doctrine. (Proof of the Apostolic Preaching. Trans. by Robinson, Armitage D.D. PDF. [NY:MacMillan Co., 1920] Par. 2. Emphasis added.)