Those men were taught by Christ Himself. They passed the faith on to other men when they formed the Catholic Church, the Pillar and Foundation of Truth.
These men pased it on to others who formed seminaries to insure that this truth was passed on faithfully.
seeing as how no protestant church can draw a lineage back to these men, the question remains.
What we DO know is these apostles and disciples WROTE volumes on what they taught. There has never been a tradition where the oral trumps the written. Both are to be in harmony.
Or are you of the school of thought which asserts the Gospels and epistles were not actually written by the original authors?
But my FRiend, the direct line back to the apostles is Pentecost as The Lord added to the church from that day on. You can find that in Acts 2 by the way.