I'm literally just straight curious. Not only is this a belief, it's a common one despite the two deities being inherently contradictory in nature and Jesus fulfilling little to NONE of the messianic prophesy. Not to mention the whole idea of Christ contradicts Judaism, and Christianity has blatantly perverted the Hebrew texts. If the deities are suppose to be the same, as Christianity seems to believe, as in they worship the Hebrew god, isn't the religion absolute pure blasphemy?
You know, I think you might have to frame it in even greater or more specific context at this point. Case and point, don't look at "the texts," or the supposed fulfillment thereof, or the contradictory nature of all the corporeal personalities, or messianic fulfillment or even the idea of blasphemy. Instead, take a good hard look at the documented personality of God. No really, read the ruminations in Isaiah or Jeremiah about wine presses and vineyards of anger, the long winded ravings that scream off the page about cutting off 10 of 12 tribes and leaving them scattered to the wind. Of flooding the earth, and generating plagues, with metaphors of stark war, always coming near or trying hard to hold itself back from promises of further division.
And think of a deity who actually would devise up a counter-being like satan or nephilim or talking snakes in the first place, understand that something may seem abnormal there.
Then maybe you'll understand what might be going on here. This is monotheism, but this is a God who instead of sharing the skies with Zeus and Thor seems to share the skies with different incarnations of himself. That in the personality there was this boundless thing that perhaps tries to hide itself from us in a fog of apophenia, yet, revealed there in the word, a Being whose mind is scarcely not on edge of being contained. Yes, God set 3 religions (or more) against each other. It is because the mind of God, which does not change, seems not to be fully decided. That is where mankind comes in for God: to help him prove a system apparently, in setting one side against the next, neither side is any longer battling a pagan deity of old (which in the OT God claims to have gave power) but now counter ideologies of God himself?