12 I did not receive it from any man, nor was I taught it; rather, I received it by revelation from Jesus Christ. -Galatians 1:12 BSB
The phrase “I did not receive it from any man” see verse 11 and its commentary.
When Paul said, “nor was I taught it,” he was talking about the gospel of Jesus Christ, who was the promised Messiah to come and of His death and resurrection. Jesus appeared to Paul and revealed Himself to him.
3 As Saul drew near to Damascus on his journey, suddenly a light from heaven flashed around him. 4 He fell to the ground and heard a voice say to him, “Saul, Saul, why do you persecute Me?” 5 “Who are You, Lord?” Saul asked. “I am Jesus, whom you are persecuting,” He replied. 6 “Now get up and go into the city, and you will be told what you must do.” -Acts 9:3-6 BSB
The word “revelation” means an unveiling. Before Paul knew who Jesus was, he had a spiritual veil over his eyes. When Jesus showed Himself to Paul on the road to Damascus, Jesus’ light blinded Paul, which induced scales to form over his eyes to indicate his spiritual condition as a religious leader who didn’t believe in the Lord Jesus Christ. His scales fell after a believer prayed over him, and he received the Holy Spirit, see Acts 9:17-19.
Paul himself understood that the only one who can take the blinders or veils off of people is the Holy Spirit, see below.
14 But their minds were closed. For to this day the same veil remains at the reading of the old covenant. It has not been lifted, because only in Christ can it be removed. 15 And even to this day when Moses is read, a veil covers their hearts. 16 But whenever anyone turns to the Lord, the veil is taken away. 17 Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. -2 Corinthians 3:14-17 BSB