Which is what the Bereans were searching to see if Rav Sha'ul was telling the truth. You seem to dismiss the Tanach.
The books of the Tanakh (Hebrew Bible) were relayed with an oral tradition passed on by the scholarly and other religious leaders of each generation, and according to classical Rabbinic interpretation, the teachings of the Oral Law are a guide to that interpretation of the Written Law which is considered the authoritative reading. Jewish law and tradition thus is not based on a strictly literal reading of the Tanakh, but on combined oral and written traditions.
Let me repeat that last part for you.
Jewish law and tradition thus is not based on a strictly literal reading of the Tanakh, but on combined oral and written traditions.
And yet the different sects of Jews still debated what was scripture and what they believed. The Jews also added other pagan derived sources such as the Talmud and the Kaballa.
And still the Jewish Priests quoted from Sirach and Tobit
And btw research shows that Council of Jamnia didn’t even happen.
Oh and I will quote again since you ignored it before.
...Paul wrote to you according to the wisdom given him, speaking of this as he does in all his letters. There are some things in them hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures. [2 Peter 3:15-16]
Quick research shows that the early Christians used the Septuagint. The books found in the Tanach plus the deuterocanonical books.