For clarity, I'm going to split your response into (2) parts ...
The average first century Christian could talk to an actual witness to Christ's life and resurrection. Ponder that for a moment.
If one of these witnesses happened to be around (in Philippi, Sardis, Antioch, Lystra, Iconium, Athens, Colossa, Ephesus, etc., ... rather than being concentrated in Jerusalem) ... no doubt.
The average first century Christian could, at least hear from, an actual witness to Christ's life and resurrection. Ponder that for a moment.
So can we (from the scriptures).
I thought the Apostles sent out witnesses to found the early churches?
So can we (from the scriptures).
And here it is again. Even with eyewitnesses to the life of Jesus, you desire that text. Though the Apostles or their immediate successors are alive and directing the Church, even though the Bible is not yet finished being written, you desire it over and above the Church that already exists.
This is the result of your sola scriptura lenses. The new Church, even though it writes and canonizes your Bible is not sufficient to teach you.
SD