Paul, an apostle of Christ Jesus by the will of God, and Timothy our brother,
“Paul was an apostle ‘by the will of God.’ Paul often would establish his credentials as chosen and sent by God because he had not been one of the original 12 disciples. Apostle means chosen and sent by God as a missionary or ambassador. By the will of God means that he was appointed; this was not just a matter of his own personal aspirations.” (Life Application Study Bible 2001)
Life Application Study Bible. Zondervan, 2011. p. 2001.