In Paul's letter to the believers in Philippi, he offers thanks for multiple generous gifts offered by these people, both to help his missionary efforts, and apparently to help the poor believers in Jerusalem.  But Paul expresses his deepest thanks not for money, not for his support, but for their faith in God.  That is what makes them his bothers and sisters: their shared belief.  He tells them that the Lord is near - that because of their faith, God is always with them.