The Jews said he wasn't. That is why his service was focused on Samaritans.

And that is because his mother wasn't a Jewess .... who's mother also was not a Jewess.

And he was married, also not to a Jewess.

What we call the OT is a product of 400 years of Priestly pandering. It's a tale, perpetrated, by their own admission, by the priests, around 620 BCE, when they "found" the books of Moses in some remote crevasse of the Temple and wanted to establish some sort of front against Egyptian pressure. Which of course was futile, as Eljakim was the first to become a vassallee of Egypt.