In writing to the believers in Galatia, Paul was concerned that they were "so soon removed from him that called you into the grace of Christ unto another gospel" (v. 6).
What was Paul's gospel that he was so concerned about? "Grace be to you and peace from God the Father, and from our Lord Jesus Christ, Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: To whom be glory for ever and ever" (vv. 3-5). Furthermore, he added that "if any man preach any other gospel unto you than that ye have received, let him be accursed" (v. 9). They were not to listen even if Paul or "an angel from heaven" (v. 8) preached another gospel.
Paul's gospel, on the other hand, was "not after man," i.e., not the sort of thing that men would make up. Human religions are all man-glorifying and God-degrading, blurring the difference between the two and tempting man with the age-old taunt: "Ye shall be as gods" (Genesis 3:5). In contrast, the gospel which Paul preached recognized man's utter sinfulness and worthlessness and rested in a glorified Christ for His finished work.
Paul had not "received it of man." This was not the tradition among his people. "Neither was I taught it," said Paul, even though he had been taught extensively in the religion of Judaism. On the contrary, he was taught the gospel "by the revelation of Jesus Christ." We can therefore not only be sure of its accuracy, but, also with Paul, join in the "ministry, which |he| received of the Lord Jesus, to testify the gospel of the grace of God" (Acts 20:24). JDM