“Taste and see that the Lord is good.” (Psalm 34:8)
The Gospels and the epistles that make up the canonized New Testament are written as historical documents. They purport to record historical events, and the things that Jesus says are recorded in the context of a chronology of events. That means the claims of the New Testament are falsifiable.
Just because the New Testament documents are written as if they are a chronicle of historical events does not mean that they are factual, of course; but the reliability of can be tested as with any testimonial claim. If all we had was a myth, we could not do that. If all we had were sayings, we could not test them for reliability.
The Gospels are written in the nature of eye witness accounts. The epistles are written in the nature of messages being conveyed first hand about events and realities that the writers personally experienced. That does not mean that they are reliable or factual, but they purport to be actual accounts of historical events and happenings that led to experiential realities in the writers of those documents.
The claims of Christianity that extend from these writings can be investigated precisely because they claim to be actual events that occurred in history as told by people who claimed to have personal knowledge of those events and happenings. As in a court of law, the credibility of the witnesses is something that can be weighed. The testimonies can be weighed against known facts and other testimonies. We can arrive at our own conclusions with some degree of certainty.
As an attorney who has litigated many cases, I can say with certainty that judges and juries do not always “get it right”, at least in this attorney’s opinion. Of course, I could be wrong. Only God ultimately knows the whole truth.
Even in the Gospel accounts, themselves, when Jesus spoke with authority and performed miracles, some believed and some did not. Even the people in the actual recorded accounts came away with different opinions. (That “honesty” itself has the ring of reliability.)
On the one hand, we were not “there” and miss the subtleties and nuances that personal, first hand observation allows in judging credibility. On the other hand, we have information that only an historical retrospect can provide.
The message is also the same now as then: Jesus claimed to be God in the flesh, urging that we should drop everything and follow Him, trust in Him. The claim is that He willingly gave up His life as atonement for our sin and, thereby, redeemed us; and He rose from the dead, thereby showing us that our redemption carries with it the promise of eternal life with God, no longer separated from Him.
The proof is not just in the writings, the testimonies of the writers and the historical facts. The proof now, as it was then, is in the testing. The Gospel can be tested in our own lives by accepting and trusting the message of Christ. The truth of the message of the Gospel has been the testimony of converts/believers for two millennium; and it is the testimony of believers today.