Abraham and all the prophets said that the Messiah would come. He did come in the first century AD, so that is when "Christianity was introduced." The reliability of the New Testament texts prove that Christianity was not a later invention.
----------------------------------------------------
....Thousands of Manuscripts
The more I read and research, the more convinced of our modern Bible translations' accuracy. Knowledge of ancient history and archaeology has encouraged me in my Christian walk. For example, I have learned that there are 5,686 ancient manuscripts of the New Testament books. (1) To put this in perspective, there are only 10 copies of Caesar's "Gallic War." There are only 7 copies of Plato's "Tetralogies." There are no more that 5 copies of any of Aristotle's ancient books. (2) It is typical to only have a handful of manuscripts of ancient books. If we reject the accuracy of the Bible, then we could not accept any of ancient history. If accept a pagan work based on only 5 copies as evidence, how could we reject a Christian book with over 5,000 ancient copies? If a person demands a different standard of proof for the books of the Bible, then he is only showing his own illogical and unscientific bias. The amazing number of copies of the New Testament books lets us check their accuracy.
For example-
Let's say you had 1,000 copies of a book at your home. Let's say someone secretly changed one sentence in one of the books. Since you would have 999 other copies of the book, you could easily spot the error.
Thousands of Quotations
In ancient times, as now, the leaders of the Christian churches would write letters to each other and often quote the New Testament within these letters. There are over 36,000 quotations of the New Testament left to us by the early Church. (3) These quotations are so extensive that from them one can reassemble the entire New Testament. (4) In this way we can check the complete contents of the New Testament from another source. The manuscripts can be checked and cross-checked. (Checked because their are thousands of copies. Cross-checked because of the extensive quotations.) This fact makes the books of the New Testament more reliable than any other books from the ancient period.
For example-
Let's say that a friend give you a book for Christmas. Your whole family loves the book and often quotes it in letters to friends. Then let's say the original book was destroyed. Happily, you quoted the book so much that your friends were able to reconstruct it word for word...............