Wasn’t Pentecost after the Lord was risen?
Yes. The sequence is: Jesus resurrects, and appears before the disciples. They get scared thinking he is a ghost. he eats fish and they are relieved because ghosts can't eat...or something like that. Then he blows on them and says "receive the Holy Ghost." (cf John 20:22)
But Matthew recounts a different story. He says that after all this, on the Pentecost, Jesus commands the disciples to meet him at Galilee. The eleven disciples show up and when they saw him there, they worshiped him but some doubted (Mat 28:17). So, we have evidence that SOME doubted all along and until the very last moment even though they saw him eat fish and even tough they received the Holy Spirit.
Obviously, not all of the disciples' eyes were opened when they saw him eat fish, or even after they received the Holy Ghost, as some have asserted here.