John Calvin Commentary 1 Timothy 2:7

"whereunto I was appointed a preacher and an apostle (I speak the truth, I lie not), a teacher of the Gentiles in faith and truth." — 1 Timothy 2:7 (ASV)

For which I have been appointed. So that it may not be thought that he makes rash assertions—as many tend to do—on a subject he did not understand well, he affirms that God had appointed him for this purpose: to bring the Gentiles, who had previously been alienated from the kingdom of God, to share in the gospel. For his apostleship was a sure foundation of the divine calling. And for this reason, he works very hard to assert it, as many received it with considerable difficulty.

I speak the truth in Christ, I do not lie. He uses an oath, or solemn declaration, as in a matter of extraordinary weight and importance, that he is a teacher of the Gentiles, and that in faith and truth. These two things indicate a good conscience, but it must still rest on the certainty of God's will. Thus he means that he preaches the gospel to the Gentiles not only with pure affection but also with an upright and fearless conscience; because he does nothing except by God's command.