How do we know the gospel is not of man?

The Apostle Paul asserts within Galatians that the gospel he preaches is not of human origin. In Galatians 1:11-12, he states, 'But I certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ.' This illustrates that the understanding and message of the gospel originate from divine revelation rather than human intellect or tradition, affirming that its truth is sovereignly given by God.