Contradiction with Matthew 10:5-6
This verse instructs the disciples not to go to the Gentiles or Samaritans, while John 12:20 shows Greeks (Gentiles) wanting to see Jesus.
Matthew 10:5-6: These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into [any] city of the Samaritans enter ye not:
Contradiction with Acts 10:28
Here Peter states it is unlawful for a Jew to associate with or visit a Gentile, whereas John 12:20 indicates Greeks approaching Philip to see Jesus.
Acts 10:28: And he said unto them, Ye know how that it is an unlawful thing for a man that is a Jew to keep company, or come unto one of another nation; but God hath shewed me that I should not call any man common or unclean.
Contradiction with Matthew 15:24
Jesus states He was sent only to the lost sheep of Israel, which contrasts with John 12:20 where Greeks seek Him.
Matthew 15:24: But he answered and said, I am not sent but unto the lost sheep of the house of Israel.
Contradiction with Romans 15:8
Paul writes that Christ was a servant to the Jews, confirming the promises given to the patriarchs, which might conflict with His interaction with Greeks in John 12:20.
Romans 15:8: Now I say that Jesus Christ was a minister of the circumcision for the truth of God, to confirm the promises [made] unto the fathers:
Contradiction with Galatians 2:8-9
Paul and Peter are seen as apostles to different groups (Gentiles and Jews respectively), suggesting a division that contrasts with John 12:20's approachability by Greeks.
Galatians 2:8-9: (For he that wrought effectually in Peter to the apostleship of the circumcision, the same was mighty in me toward the Gentiles:)