I'm reading Trent Horn right now, and it seems to me the Catholic objection to Sola Scripture is mostly sophistry. Protestants claim everything should be tested based on what's in the written scripture. It's not that tradition has no role, it's just subordinate to apostolic teaching. The real difference is our definition of "apostle", which for Catholics includes people other than the original 12 + Paul.

So wrt to your question above, the early Christians trusted the apostles, the people who 1. were eyewitnesses, and 2. were directly sent out by Jesus. We continue to trust those same people, with the help of human reason applied in faith down the centuries.