Paul says he thanks God because he hears of Philemon’s “love and of the faith” toward the Lord Jesus and “for all the saints.” He also says the saints’ hearts “have been refreshed through you,” setting a relational context for the request that follows (Philemon 1:4–7).
Paul tells Philemon that he regularly thanks God for him and keeps mentioning him in prayer (Philemon 1:4). The reason is what Paul has been hearing about Philemon’s love and his loyalty toward the Lord Jesus and toward God’s people (Philemon 1:5). These verses sit right after the letter’s opening greeting (Philemon 1:1–3) and before Paul’s main request about Onesimus (Philemon 1:8–16). Paul begins by voicing gratitude and prayer rather than jumping straight to the problem he wants addressed.
Paul presents Philemon as someone whose life is marked by both love and faith (verses 5, 7). Explicitly, Paul says he keeps thanking God and keeps bringing Philemon up in prayer (verse 4). The immediate reason for this gratitude is what Paul “hears” about Philemon (verse 5), suggesting Philemon’s reputation is known beyond his own house gathering.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Philemon 1:4-7, then read the surrounding section so the answer stays connected to the passage's flow.
4I thank my God always, making mention of you in my prayers,
5hearing of your love, and of the faith which you have toward the Lord Jesus, and toward all the saints;
6that the fellowship of your faith may become effective, in the knowledge of every good thing which is in us, in Christ Jesus.
7For we have much joy and comfort in your love, because the hearts of the saints have been refreshed through you, brother.