"Joyfully pressing toward the mark" is one of many titles that could be ascribed to this epistle.  Paul clearly had a deep affection for the believers there and his gratitude is stressed throughout.  He wrote to encourage and thank them and in this he stresses various concepts of 'joy' when looking to keep them motivated rightly.  We find the founding of the church in Philippi mentioned in Acts 16.  The two converts that are mentioned are Lydia and the unnamed Philippian jailor.  It's interesting to consider that Paul wrote this epistle while imprisoned in Rome realizing he had spent time in prison while in Philippi.  Paul will stress the 'benefits' that came with his imprisonment (1:13).  His gratefulness and faith-based optimism are throughout these chapters (even though in prison).  The latter part deals a great deal with what they should be thinking and how they should be thinking.  A Christ-centered focus will be essential in maintain proper perspective throughout life.