Has the New Testament Been Corrupted?

What do we know about the New Testament books, when did they begin to be regarded as authoritative, and have they been corrupted over time? 1 Timothy 5:18 quotes a saying from the gospels as Scripture. 2 Peter 3:15,16 characterizes Paul's writings as Scripture. Now I realize the authorship of these books has been questioned, but it represents the understanding of the early church. Clement of Rome appeals to and quotes Paul's letter to the Corinthians as given under the inspiration of the Spirit. Polycarp mentions Paul's writing to the Philippians with special wisdom and quotes him as Scripture. Justin Martyr mentions the memoirs of the apostles, called gospels, and frequently quotes them. Irenaeus states there are four gospels, Matthew, Mark, Luke, and John, and says there can be no more and no less and gives passages from them. He also references the letters of Paul. Also, Papias is quoted by Eusebius as attesting to the four gospels and their authors. The Muratorian Canon, though damaged, lists the bulk of the New Testament books. Also, we have large numbers of manuscript copies of the New Testament, many very early, and many early translations, along with multitudes of quotes in the early church fathers.

While there were controversies over whether some books belonged in the New Testament and there were church councils that later put out official degrees as to what should be included, the basic content of the New Testament was never up for grabs. As for the contents, the preservation of the New Testament is remarkably better than any book of even remotely comparable antiquity. While there is a certain degree of textual corruption, it is peripheral and the substance of the text is very clearly preserved. But does the New Testament really reflect the beliefs of the early Christians? Given the early attestation of the books, are we to believe Christians for some reason changed their beliefs, beliefs they were quite early willing to die for? Also, from quite early, Christianity had critics. Could Christians have changed what they believed without them noticing it? Could someone have deliberately changed the New Testament? The most common suspect for this is Constantine the Great. But we have manuscripts with large portions of the New Testament from before Constantine and all of the books very soon after. Also, all the authors mentioned above were before Constantine. Constance had no chance to alter the New Testament. Also, it is difficult to see who before Constantine would have been able to do so. The church was scattered and persecuted, and we have large numbers of manuscripts over a wide geographic area. It is unclear how a person could have managed to change them all and suppress the older versions. Now I realize this does not  prove Christianity is true. Rather, it is an attempt to show the New Testament is an accurate record of Christian claims and beliefs. They need to be accepted or rejected on that basis.