Hello Anna,
Great question!
Over the first century, many people wrote about Jesus, what He taught, what He did, and how His followers should live. These authors wrote in various cities and at various times. Sometimes they knew of each other and referenced the existing works; sometimes they worked independently.
Over time, all these writings were collected and bound into one volume. This book we call the Bible is not a monolith work of one author, but a collection of dozens of authors.
The works collected to form the New Testament were collected precisely because they provide the best evidence and best witness to Jesus.
I understand your concern for using extra-biblical sources, and indeed we use these, as well. As I mentioned above, they provide a great deal of corroborating evidence to Christianity’s claims.
But we have no reason to reject the works collected in the Bible. They were selected because they compile the eye-witness evidence for Jesus. These are precisely the documents we should consult first, not the ones we should exclude from the conversation.
Now, if the documents had been changed over time, or they were written so far after the fact that they have no real relevance, then we’d have a reason to exclude them. But neither seems to be the case. All the evidence of history points to the New Testament accounts being early and accurate, unchanging throughout time.
Thanks for the question!