Throughout the gospels and Acts, we see Jesus proclaiming that the 12 apostles will be Jesus’s witnesses to the world; Jesus prophesied that they would teach the world of his life, ministry, death, and resurrection.
This is relevant because it is highly unlikely authors would be attributing overtly false predictions to Jesus. The authors at least believed the apostles were actively teaching, and their belief on the matter would likely be justified rather than random. And being a witness, of course, involved testifying of Jesus’s fulfillment of Messianic prophecy, his teachings, His death, and His resurrection.
Peppered throughout the book of Acts are rich and loud proclamations of the apostles working essentially full-time to teach about Jesus’s life, ministry, death, and resurrection.
Consider these verses:
This is relevant because Acts is the historical document, written during the life of the witnesses, which records how the apostles of Jesus proceeded to live after his crucifixion. And, as seen above, it is peppered with references to the apostles acting as witness-teachers of Jesus’s life, ministry, death, and resurrection.
But so what? Plausibly...
• Acts 6:2 — “Now at this time, as the disciples were increasing in number, a complaint developed on the part of the Hellenistic Jews against the native Hebrews, because their widows were being overlooked in the daily serving of food. So the twelve summoned the congregation of the disciples and said, “It is not desirable for us to neglect the word of God in order to serve tables. Instead, brothers and sisters, select from among you seven men of good reputation, full of the Spirit and of wisdom, whom we may put in charge of this task. 4 But we will devote ourselves to prayer and to the ministry of the word. The announcement found approval with the whole congregation; and they chose Stephen, a man full of faith and of the Holy Spirit, and Philip, Prochorus, Nicanor, Timon, Parmenas, and Nicolas, a proselyte from Antioch. 6 And they brought these men before the apostles; and after praying, they laid their hands on them. 7 The word of God kept spreading; and the number of the disciples continued to increase greatly in Jerusalem”
• Eric Eve (summarizing Gerhardsson's work): “speaking in the name of Jesus just as the rabbis’ disciples would speak in the name of their masters. Indeed, the Apostles devoted themselves to the service of the word, leaving mundane tasks to others (Acts 6.1–6), rather as the rabbinic ideal was to devote oneself to the study of (and obedience to) the Torah. This service of the word included not only teaching it, but also intense discussion to discover its full meaning. In this context ‘the word’, as an analogue to the rabbinic oral Torah, referred primarily to Jesus’ sayings and deeds, as summarized in the speeches in Acts (Acts 2.22–36; 3.12–26; 4.8–12; 5.29–32; 10.34–43), although the Church also took over the Scriptures of Israel. In Luke’s presentation Jesus laid the foundation for its Christian interpretation (Luke 24.27, 32, 44–45), and the work of continuing this interpretation was carried on by the Apostles.” Citing {Gerhardsson, Memory, 208–61.}
We read both in 1 John and 1 Peter assertions that the apostles act as witnesses to Jesus's life and ministry.
This is relevant because these, along with the other sources above, all converge on the same truth, with no evidence of dissent.
Irenaeus, writing in c. AD 185 records how Clement (c. 30-100 AD) was a disciple-student of the apostles, being specifically taught by them.
This is relevant because this very specific belief of Clement’s an Irenaeus’s is likely not a complete shot in the dark, but rather is likely to enjoy some measure of justification. (They were up close to the events.) It is also one of the few resources, aside from Acts, which speak on the matter.
But so what? Plausibly...
The Jerusalem church pass-down Jesus-bio, delivering to their successors and even other churches.
This is relevant because the apostles lead the Jerusalem church, and the apostles passing down this content in part explains the fact that the Jerusalem church passed down Jesus-biography.
In the 1st century, while witnesses were still alive, the number of people converting to believing in Jesus's claim to be Messiah as well as his resurrection was massive. This is relevant because it is near impossible to explain if Jesus’s own apostles abandoned the faith or were otherwise lacking zea in their teaching of others. The growth is far easily to explain if they apostles were actively teaching.
The earliest Christians honored the “The Twelve” (apostles) as leaders of all Christendom, as long as those apostles were still alive and active.
This is relevant because it is hard to explain unless the apostles were witness-teachers.