Jamieson, Fausset and Brown on Philippians 1:5

5. Ground of his "thanking God" (Php 1:3): "For your (continued) fellowship (that is, real spiritual participation) in (literally, 'in regard to') the Gospel from the first day (of your becoming partakers in it) until now." Believers have the fellowship of the Son of God (1Co 1:9) and of the Father (1Jo 1:3) in the Gospel, by becoming partakers of "the fellowship of the Holy Ghost" (2Co 13:14), and exercise that fellowship by acts of communion, not only the communion of the Lord's Supper, but holy liberality to brethren and ministers (Php 4:10, 15, "communicated ... concerning giving"; 2Co 9:13; Ga 6:6; He 13:16, "To communicate forget not").