"Again we should not regard this as mere literary flourish .... Paul must have had an extensive prayer list and presumably spent some time each day naming before God all his churches, colleagues, and supporters. This would help maintain and strengthen the sense of a faith shared with "all the saints" (5-6)." [1][source]
Whenever Paul remembered Philemon in prayer he gave thanks for him. Evidently his testimony had been consistently honoring to the Lord. The basis of this thanksgiving was Philemon"s love and faith. Reports of these qualities had undoubtedly reached Paul through Epaphras ( Colossians 1:7-8) and probably others as well. The objects of Philemon"s love and faith were "all the saints" and "the Lord Jesus" ( Philemon 1:5) respectively. The Greek construction is chiastic (cf. Ephesians 1:15; Colossians 1:4). That Isaiah , the first and fourth elements in Philemon 1:5 go together, as do the second and third. This construction emphasizes the unity of the entire thought: love for the saints grows out of faith in Christ. [source][source][source]