"Your remembrance of me, expressed in aid to me, is your fellowship in the Gospel. I labor in the Gospel. But because you, by helping me, give greater freedom to the Gospel, you too participate in the Gospel."
"The Philippians began to send aid to the Apostle as soon as he departed from Philippi. This is what he means by saying, 'from the first day.' They sent to Thessalonica, they sent to Corinth, and now to Rome.
Thus it turned out that they truly helped the Apostle up to the very hour when he wrote the Epistle. This is why he says, 'until now.'"
According to blessed Theodoret and some others, by "fellowship in the Gospel" one may understand participation in the work of redemption in general. With respect to the Philippians, this may mean both their appropriation of the fruits of redemption and their ministry to others in this appropriation.
This thought is expressed more clearly in verse 6, where two chief thoughts are observed. First, the work of salvation is accomplished by God. It begins by Him, continues by Him, and is completed by Him.
In other words, the work of salvation is the work of God's grace. The second thought is that the work of salvation does not end with a person's death, but continues even afterward, even until the day of Christ.
By this is meant "the last day, which no night cuts short," as St. Basil the Great said. St. John Chrysostom sees in this verse an indication of the relation between grace and freedom.
This is shown even in the words, "God will complete it." "Again there is praise for the Philippians, because they drew to themselves the grace of God, which helped them rise above human nature."
"If God will complete it, then not much labor will be needed on our part. We must hope that, with His help, we shall accomplish everything more easily."