“For it is only right for me to feel this way about you all because I have you in my heart, since both in my imprisonment and in the defense and confirmation of the gospel you all are partakers of grace with me.”
Paul was confident God was at work in the Philippians and would complete the work because he (Paul) was deeply connected to them. They were working together in the gospel (with God’s help and grace); and they were pulling for Paul in his imprisonment. We learn later that one of the Philippians (Epaphroditus) was sent to help Paul and he almost gave his life doing so because he was sick (Phil. 2:25-30). So with God’s leadership, Paul and the Philippians were serving together and this gave Paul great confidence in their salvation and God’s ongoing work in their lives.