Paul Received His Knowledge and Understanding of God's Ways from God Himself, Not From Human Mouths (Galatians 1:11-17).
‘For I make known to you, brothers, as touching the Gospel which was preached by me, that it is not after man. For I neither received it from man, nor was I taught it, but it came to me through revelation of Jesus Christ.'
Paul stresses the special revelation that he received. It is not something that he has learned from others. It came direct from God. It was revealed to him by Jesus Christ. It was not man made, but God-made. Others have learned what they have learned from men, but he had received what he taught directly from the Prime Source, the risen Christ.
‘Through revelation of Jesus Christ.' He first received this revelation very vividly on the road to Damascus when the risen Christ appeared to him (Acts 9). The appearance of the risen Christ had given new meaning to all that he had previously known and learned about Jesus, and that must have been a considerable amount, even if received in slightly distorted fashion, given the circumstances that he had been an direct opponent of the Gospel and had put many Christians to questioning and torture. But then he had had further vivid experiences with God, experiences which were beyond the norm. See for example 2 Corinthians 12:2-4 which was almost certainly biographical. And through those experiences the truth was made known to him when he was alone with God. We may possibly compare how God spoke with Moses face to face as a man speaks with his friend (Exodus 33:11), although Paul may well have received his revelation through thoughts in his inward mind.
And if they wish to they can confirm this themselves by considering his past.