@SatyrService: I am not sure the point you are trying to make in regards to there being many preachers. Yes, there were many preachers at the time and there are many preachers now. I do not see the relevance to your argument.
We do not use the scientific method to study history. I would be curious to know what test with what variables and how many trials you use to prove history. Try to prove George Washington using science.
I am not sure what physical evidence you are referring to. Historians use the criterion of embarrassment to establish truth. If you are not familiar with this - it means if I say something in such a way that makes it less likely to be believed but not less likely to be true then it is less likely to be true. The reason is if I was trying to just get attention (or whatever my motive was) I would craft it in such a way that it would be at the optimal believability level. For example, Jesus grave was found by women - not men. This does not make it any more or less likely to be true as women are not any stupider than men. But - since this time period was sexist - it is less likely to be believed. If the writers of the New Testament had a personal agenda they would try to make the story as believable as possible and would not undermine the "credibility" of the story by saying it is found by women. Historians also look at the number of people who independently claim something happened. Today the Bible is one book, but it is actually a collection of many books.
Even if the earliest versions were 150 CE - that is still within the 150-200 year time period that historians usually consider reliable when studying ancient history. And still much better than many other ancient texts, such as Herodotus. They existed in the languages of Greek, Latin, Syrian, Slavic, Gothic, Ethiopic, Coptic, and Armenian. The earliest fragment comes from the Gospel of John which is around 125 CE. If you are interested in studying this further I will refer you to The New Testament: A Historical Introduction to The Early Christian Writings by Bart Ehrman. John is generally dated to have been written between 90 CE and 110 CE which means this copy was anywhere from fifteen to thirty-five years old. For reference to the dating of the Gospel of John I suggest you look at Gospel According to St. John: Black's New Testament Commentaries by Andrew Lincoln.
I apologize for the length of this response. Please let me know if you want me to clarify anything further.