Basically, the question of authenticity of Scripture is a good question.
Jewish tradition has always placed the greatest importance on the validity of their scriptures. Jesus Christ taught that not one jot or tittle in Matthew 5:18 shall pass away until heaven and earth pass. Jesus understood the sanctity of Jewish scriptural text according to Jewish tradition if even one dot is changed in the Talmud then that edition of the Talmud is no longer a valid copy and must be buried.
Unfortunately Christian tradition is much different. Until the first Council at Nicaea by the Emperor Constantine there was no central clearinghouse on Scripture. The apostles themselves did not write the Gospels, people who were close to them wrote what they remembered the apostles speaking about as they traveled from church to church. It is amazing that the four Gospels are as close in content to each other as they are considering where they came from. Further, it is amazing that so many variations of the Gospels that still exist are relatively close in what they say to each other. After the first Council of Nicaea and after a second Council of 385 nearly all of the books that the early Christian church used that were not approved by these councils were collected and destroyed.
After the third century we start having some unity and a central clearinghouse for Scripture. From the third century on the Scriptures maintained by the Roman Catholic Church are fairly reliable. Even after the schism of the Eastern Church and the Roman church the two “universal churches” mostly agree on scriptural content.
While the epistles of James and Paul were written in their own hand supposedly, we do not have any of the original paper that they were written on. Those epistles sent to a particular church were copied and copied nearly ad infinitum so that everyone got to read the words of Paul to a lesser extent James. There were likely many more epistles that we no longer have. There were perhaps other Gospels, in modernity there have been some “Gospels” that have shown up but their authenticity is in doubt.
Regardless of the perfection of the scriptural record that we have today that has survived antiquity we can be sure that the Old Testament at least where it agrees with the Jewish text is still good. When reading the New Testament it is wise to seek spiritual guidance. Christ has told us that the Holy Ghost will reveal truths to us, revelation if you will to the authenticity of what you read.
Many churches consider every letter of the Bible to be holy, much like Jewish scholars and their tradition of the Talmud. It is fascinating to note that although many congregations or denominations consider every word of the New Testament and the Old Testament to be the word of God they interpret those words dramatically differently and so we have thousands of Christian denominations in the world. I don’t begrudge any of the denominations, my hope is they all love the Lord and Savior Jesus Christ and are doing their best to serve Him. Further it is my hope that the Lord will recognize the efforts of these people and trying to serve Him until we all come to the unity of faith.