2 Timothy 1:1

Here is, I. The inscription of the epistle Paul calls himself an apostle by the will of God, merely by the good pleasure of God, and by his grace, which he professes himself unworthy of. According to the promise of life which is in Christ Jesus, or according to the gospel. The gospel is the promise of life in Christ Jesus; life is the end, and Christ the way, Joh 14:6. The life is put into the promise, and both are sure in Christ Jesus the faithful witness; for all the promises of God in Christ Jesus are yea, and all amen, Co2 1:20. He calls Timothy his beloved son. Paul felt the warmest affection for him both because he had been an instrument of his conversion and because as a son with his father he had served with him in the gospel. Observe, 1. Paul was an apostle of Jesus Christ by the will of God; as he did not receive the gospel of man, nor was taught it, but had it by the revelation of Jesus Christ (Gal 1:12), so his commission to be an apostle was not by the will of man, but of God: in the former epistle he says it was by the commandment of God our Saviour, and here by the will of God. God called him to be an apostle. 2.

— Matthew Henry's Concise Commentary (public domain)