Paul thanks God because of the Philippians’ “partnership in the gospel” from the first day until now: Philippians 1:3–5. He links this gratitude with joy and with confidence that God will finish the work he began in them: Philippians 1:6.
Paul tells the Philippians that remembering them leads him to thank God. His prayers for them are marked by joy because of their long-running partnership in spreading the message about Jesus, from the beginning of their relationship up to the present. These lines come right after the letter’s opening greeting and set an emotional tone of gratitude and warmth (see Philippians 1:1–2). Paul reports what he does when he thinks of them (thanks, prayer, joy) and then explains why (their partnership).
Paul describes a relationship shaped by gratitude, joy, and long-term cooperation. He thanks God whenever he remembers the Philippian believers (verses 3–4). His joy is not presented as vague optimism; it is tied to something specific: their ongoing “fellowship” aimed at advancing the gospel (verse 5).
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 Philippians 1:3-5, then read the surrounding section so the answer stays connected to the passage's flow.
3I thank my God whenever I remember you,
4always in every request of mine on behalf of you all making my requests with joy,
5for your fellowship in furtherance of the gospel from the first day until now;