Ancient Christians adopted some of the Jewish scriptures into their own scriptures and eventually called that the Old Testament.
The 27 books that comprise the New Testament were selected out of a much larger group of writings. In the early days, there was no official set of scriptures. Different Christian groups had different writings they considered to be scripture. It is estimated that in ancient Christianity, there were as many as 200 writings that, at least by some Christians, were considered to be scripture.
Later, official scriptures were developed when people got together and decided which of these to put in and which to leave out. No one knows exactly how they decided, but sometime in the late 300’s, “official” Christianity decided on 27 books.
So what is “official Christianity”?
In the early 300’s, the Roman Emperor Constantine decided he could use Christianity to unify the far-flung Roman Empire. The problem was, though, that Christianity itself was not unified. There were many different Christian groups with many different beliefs.
Constantine approached influential men in some of these groups and told them that if they would get together and decide on one “official” version of Christianity, he would make Christianity an official institution inside the Roman government. Christianity, as a religious institution, would be an official arm of the Roman government, and these men would be at its head.
They accepted Constantine’s offer, and Christianity as an institutional religion was born. As Constantine directed, they got together and began deciding official Christian beliefs. This would later become the only “correct” version of Christianity, backed up by the power of the Roman government.
There were many different versions of Christianity back during the early days. Out of all those, they chose one and declared it to be the only “correct” one. Then, out of all the different Christian writings that were around, they chose only those that reflected what they decided was the “correct” version of Christianity. The rest they rejected, and the Roman government outlawed them. Eventually, many of these writings were destroyed.
The New Testament contains the books they selected. The books they selected support what they decided was the “correct” version of Christianity. They threw out what did not support it.
If that’s the way the New Testament was put together, then how can we view the Bible as in any way Christian scripture? After all, the motives or the people who selected them were not pure. They had their eyes on the wealth and power that would result from them being part of an institutional religion that was a part of the Roman government.
You can see the New Testament as scripture by believing that, whatever might have been the motives of the people who selected them, God was in some way at work in the process. Maybe they didn’t include all of what they should have. Maybe they tossed out a lot that should be in there. But God at least made sure the minimum of what we need survived.
That’s something you just have to take on faith. Either by working through them or working in spite of them, God made sure we would have at least the minimum of what we need.
That’s how, regardless of how the books were chosen, you can look at the New Testament as Christian scripture.