Paul, an apostle of Jesus Christ by the will of God, according to the promise of life which is in Christ Jesus,
This letter is the last testament and swan-like death-song of Paul (Bengel).
According to the promise of life which is in Christ. Paul's apostleship is in order to carry into effect this promise (cf. Titus 1:1-2). This "promise of life ... in Christ" (cf. 2 Timothy 1:10; 2 Timothy 2:8) was needed to nerve Timothy to boldness in undertaking the journey to Rome, which would be attended with much risk (2 Timothy 1:8).