The Apostle Paul
Philippians 1:1ESV·traditional attribution

Paul and Timothy, servants of Christ Jesus, To all the saints in Christ Jesus who are at Philippi, with the overseers and deacons:

John Calvin Reformed @genevareformer

1 Paul and Timotheus, servants of Jesus Christ While Paul is accustomed, in the inscription of his epistles, to employ titles of distinction, with the view of procuring credit for himself and his ministry, there was no need of lengthened commendations in writing to the Philippians, who had known him by experience as a true Apostle of Christ, and still acknowledged him as such beyond all controversy.

Matthew Henry Presbyterian @wholebiblehenry

We have here the inscription and benediction. Observe, I. The persons writing the epistle - Paul and Timotheus. Though Paul was alone divinely inspired, he joins Timothy with himself, to express his own humility, and put honour upon Timothy. Those who are aged, and strong, and eminent, should pay respect to, and support the reputation of, those who are younger, and weaker, and of less note.

Commenting on Philippians 1:1-2

John Gill Reformed Baptist @doctorgill

Paul and Timotheus, the servants of Jesus Christ,.... The apostle sets his own name first, as being not only superior to Timothy in age, in office, and in character, but the sole writer of this epistle. The reasons of his joining Timothy with him are, because he was with him when he first preached at Philippi, and so was known unto the Philippians, and respected...