Who were they?
Three disciples (Matthew, John, Peter), two of the Lord’s brothers, (Jude, James), Paul, the Roman Citizen and former persecutor of the Church, specifically chosen by Christ as his messenger to the Gentiles and their Kings. Then we have Luke, the traveling companion of Paul, who it is believed was one of the 70, a Hellenistic Jew, and relative of Paul. The opening passage in the New International Version says that Luke “investigated everything from the beginning,” but it is better translated as ‘came alongside everything at a very early stage‘. There is a strong case to be made that Luke could be the same as Lucius of Cyrene, one of the teachers at Antioch, along with Barnabas.
Then there’s the anonymous author of Hebrews, who I believe was Barnabas, writing along with, and under the authority of Paul. There are many possibilities, but Tertullian seems to have known from early tradition that it was him.
Finally there’s Mark, the translator/ interpreter of Peter, who wrote the sermons of Peter into what we know as the Gospel according to Mark.
Peter, James and John were known as the ‘pillars’. James, the brother of Jesus, replaced James the brother of John after his execution in 41 A.D.