Why should I believe that the gospel accounts haven’t been changed in 2000 years?

The four gospels about Jesus in the Bible were written down by eye witnesses and people who interviewed them. They knew that oral accounts change; the point of writing something down is to preserve it. We have more copies of these early manuscripts about Jesus than we have about people like Julius Caesar from the same era. They were written while the church was persecuted, so there was nothing to gain by making the story up. For details of why we can trust our reconstructions of the original manuscripts, see Myths and Mistakes in New Testament Textual Criticism by Elijah Hixson and Peter Gurry, and more widely He Walked Among Us: Evidence for the Historical Jesus by Josh McDowell and Bill Wilson. For the authenticity of the Books of the Law in the Old Testament, see The Books of Moses Revisited by Paul Lawrence.