One of the reoccuring themes in Philippians is "fellowship." Paul is thankful to God for the Philippian beleivers and prays for their continuance in the faith. Part of the joy that Paul experienced is their 'fellowship in the Gospel.' Fellowship is 'real spiritual participation' (Jameison, Fausset, and Brown). I must admit that when I think of 'fellowship' I think of getting together and having fun along with some type of food. That is more like feasting, but it does not necessarily mean 'fellowship.' Fellowship, therefore is deeper than just getting together, it is deeper than feasting. It is sharing in something and not just of something. In this verse Paul was in prison, and yet he rejoices when he thinks of the Philippian believer's fellowship in the Gospel. Fellowship is a spiritual participation, being commonly involved in the spreading the good news of Jesus Christ. In this case, the Philippian church among other things, gave financially to Paul. They were sharing in this endevour to spread the Gospel.
How about it dear saint? Do you have an active spiritual participation in the Gospel? It may be in giving to your church's missions program, it may be in passing out Gospel tracts, it may be in going out on visitation, or door knocking, it may be in sincere prayer, it may be in speaking to others about the Lord Jesus Christ - speaking to family members, co-workers, neighbors, friends, even enemies. I do believe it is all that and more. How goes your 'fellowship?'