We have a lot of written information such as the book of Acts and the epistles. The Apostles, however, lived within this time period and knew of which they spoke.
There is one thing I am committed to focusing on in this blog, and that is the fact that these apostolics did not have New Testament writings.
Most of the Apostles studies were from the Hebrew Bible itself which consists of the Torah, the Writings, and the Prophets ... what we know today as the Tanach.
The Apostles also had commentaries from teachers like Hallel (Pharisees), Shammai (Sadducees) and Gamliel who Paul sat under. These teachings and traditions found their way into many NT writings. The Apostles commentaries were known in the Greek as paradosis or 'a giving over which is done by word of mouth or in writing.'
(2 Thessalonians 3:6)
These teachings ... oral traditions ... started with Adam and were passed down orally from generation to generation. I have a friend that can trace his lineage back 3500 years in the line of the Levitical priesthood. To this day a Levitical priest cannot touch a dead body. And where do you think washing hands before saying a blessing comes from?
Here is a challenge for you...try going back to the Torah (Torah, meaning 'instruction'), the first five books of the Bible, and look for Messiah. Not in the places you have already been taught about, but in other places.