When the bible was written and the first people read it, why did they assume that its content was the truth? Couldn't the author just have convinced a group of people that this was the truth and then the group spread the believe around the world?
Just curious not offensive.
One major point to consider is that there isn't just "one" author.
First and foremost, the "Bible" is a collection of 66 (or 73) discrete books written by at least 40 different authors over a period of over 1500 years. To say that at least 40 authors over 15 centuries all had the same hidden agenda is conspiratorial thinking at its best.
The Old Testament
Clearly some of the oldest books (Genesis) rely on some oral tradition. If the charge of "it was never intended as fact were to be leveled, it would be here. Here, there is some spectrum of belief on whether it is intended to be literal.
Much of the remainder of the Torah (2nd half of Exodus, Deuteronomy, Leviticus) is legal in nature, and thus not subject to "whether or not it happened."
Likewise, the vast majority of the Prophets are writing sermonic warnings about things that hadn't happened yet - but archeology has shown that these things did happen. Be it Israel, Judah, Moab, Ammon, Midian, Assyria, Babylon - the fall of these nations are all facts proved by archeology. The sermonic warnings to be taken from them really aren't "history" in any event.
The "Writings" (Job, Psalms, Proverbs, Song of Solomon) were typically considered to be what we would call fiction today (Job is questionable) - but have always been interpreted as such.
The New Testament
Most of the books in the New Testament, by contrast, were written by eyewitnesses. These books were all written within 70 years of the death of Christ - at the latest, and many scholars believe the vast majority was complete within 40 years of the Crucifixion. These books do claim to be purveying an "orderly" eyewitness account of things "they saw and heard." Indeed, the book of Acts claims at least 500 witnesses to these things. The overall tenor of the Gospel accounts at a bare minimum suggest that these were true.
The Epistles are all letters being communicated from one writer to another, and the expectation is clear that these are events the authors believe occurred.
The point is that each work needs to be considered on its own, and that the New Testament is more "historical" than Old, in terms of the standard of evidence that modern historians would accept. That said - I would argue the most important and audacious claims - namely that God became a man, died on cross, and then wasn't dead - are in the New Testament. By contrast, the Old Testament miracles have nothing to suggest they aren't true - but their importance is less than that of the Crucifixion.
Ultimately, each person decides the historicity of each work for themselves - but there is nothing to suggest that any of the authors were being deceptive. Some were clear they weren't writing "history" (indeed, that concept comes after the canon was closed). Some works were intended as story. Some works were intended as law. Some works were intended as prophecy.
But the most vital claims of the Bible made by Christians that are story in form were written by people who clearly believed it. Indeed, most of them went to their martyr's graves maintaining it. That pattern continued for another 300 years. That seems like an awfully high price to pay for something you were making up - or hadn't experienced in faith yourself.
When the bible was written and the first people read it, why did they assume that its content was the truth?
For the New Testament there is a simple answer to this question:
The people who first read it where in a good position to either already know or able to verify the veracity of the claims made in the text.
Once the most extraordinary are confirmed trust in the rest comes easily enough.
