21 Salute every saint in Christ Jesus. The brethren which are with me greet you.
[AD 384] Ambrosiaster on Philippians 4:21
The greeting of the brothers is an act of mutual consolation. It calls to mind those who have been split apart.

[AD 407] John Chrysostom on Philippians 4:21
"Salute every saint in Christ Jesus." This also is no small thing. For it is a proof of great good will, to salute them through letters. "The brethren which are with me salute you." And yet thou saidst that thou hast "no one like-minded, who will care truly for your state." How then sayest thou now, "The brethren which are with me"? He either saith, "The brethren which are with me," because he hath no one like-minded of those who are with him, (where he doth not speak of those in the city, for how were they constrained to undertake the affairs of the Apostles?) or that he did not refuse to call even those brethren.

[AD 458] Theodoret of Cyrus on Philippians 4:21
Not everyone who calls himself a saint is a saint but only the one who believes in the Lord Jesus and lives according to his teaching.