Hearing of thy love, and of the faith … saints. He would hear of these instances of Philemon's faith and love naturally through Epaphras (see on Philemon 1:2). Refer "faith" to "the Lord Jesus" and "love" to "all the saints" (a chiasmus, or cross-reference).
Note that the phrase is πρὸς (i.e. erga, towards) τὸν κύριον, but εἰς (i.e. upon) τοὺς ἁγίους; perhaps because Christ cannot now be reached by bodily efforts, but only aspired towards by the soul; while the poor can actually be reached and ministered unto.
"Ye have the poor always with you, but me ye have not always" (Matthew 26:11). All Christians are called "saints" in the Scriptures, as Ephesians 1:1, and invariably. What a reminder to them of their "holy calling" (2 Timothy 1:9)!
Meyer notes, however, that it is not uncommon with St. Paul to vary the preposition (Romans 3:20; Galatians 2:16).