Rejoicing over Christ's Work in Others Begun and Completed

What makes the apostle rejoice so much over the Philippians? In these four verses, the apostle expresses abounding joy over the Philippians’ receipt of the gospel, growing in the gospel, and participation in his own ministry of the gospel—most of all because he is laser-focused upon the Day of Christ, and these are all indicators of a begun-work that will be gloriously completed in that day!