How do we know Paul is an apostle?

Paul defends his apostleship in Galatians, declaring that he is 'not of men, neither by man, but by Jesus Christ and God the Father' (Galatians 1:1). His authority comes directly from the revelation of Jesus, who called him to be an apostle. He recounts this divine encounter in his other writings, affirming that he did not receive the gospel from men but received it through revelation (Galatians 1:11-12). This underscores the divine mandate behind his message, setting him apart from others who claimed authority.