because of your partnership in the gospel from the first day until now.Previously, Paul has told us that he is thanking God for these Philippian Christians. He does this on every remembrance of them, and he does this with joy! Now he is telling us a specific reason why these Christians turn Paul toward thankful, joyful prayer.
(Philippians 1:5, ESV)
Paul says that it is because of their "partnership in the gospel" that he is exulting in joyful prayer. The Greek word that is translated "partnership" is very much a business word. It is the same word that is used to describe two "partners" going into business together.
In a very real sense, these Philippian Christians had gone into business together: The Gospel Business. The church at Philippi had shown support to Paul more than once. They had even recently sent Epaphroditus (one of their members) to visit Paul in prison. Roman prisons wouldn't always take care of their prisoners so well, so a prisoner would be dependent on friends and family to meet their needs. The Philippians knew this and sent Epaphroditus along with some supplies.
So Paul is thanking them for this genuine partnership in the gospel.
An important lesson to begin to learn from this is that the work of the gospel is not to be left for the apostles. It is a work for the whole church to partner in.