"Always in my every supplication for you all, making supplication with joy.  In view of your participation in the gospel from the first day until now, having become confident of this very thing: the One having begun a good work in you will complete it until Christ Jesus' day."

Paul goes on to tell the Philippian church why he is so thankful for them.

Your participation in the gospel.  How do you participate in good news?
A completed good work.   How do you participate in God's completion of you?

These verses contain one of my favorite phrases; a phrase that is repeated many times in Philippians:  In the Greek it is "Pantov Imon" and it is translated "all of you".

Always in every supplication (or prayer) for ALL OF YOU...

For whom is Paul praying?  Who participates in the gospel?  Who belongs to the church?  ALL OF YOU.

This is a phrase for anyone who has ever felt small, or doubted their worth in the greater body.  Christ will complete this work in ALL OF YOU.