Paul is affirming that his apostleship does not have a human origin; he likely says this in response to claims that it did.1 He is not an apostle because he was appointed such by a group of men (e.g., the Antioch church council, see Acts 13:1–2). He is not an apostle because he was appointed by an individual (e.g., Barnabas, see Acts 11:25–26; Ananias, see Acts 22:15).2
1 Paul, apôtre, non de la part des hommes, ni par un homme, mais par Jésus-Christ et Dieu le Père, qui l'a ressuscité des morts,