Why dont Jews believe in a trinity? I would think that the voice of the people that wrote the old testament would carry some weight and be taken into consideration by christians?
JoJo....The Trinity doctrine was a later formulation, one that did in some ways offer a harmonization of various texts. It did not however actually represent any previous conception of God. As has been mentioned a number of times, scores of passages in the OT and many intertestamental works conceived their God as of an unapproachable infinitude that worked indirectly with humans and creation through the agency of an emanation or aspect/virtue of himself.
Philo of Alexanderia famously honed the idea, but it was centuries older in some form.
For instance Philo explains the Genesis 1:26 passage where God says Adam had become 'like one of us' as referring to his plurality of 'virtues' but are likened to 'beings'.
The expression, "one of us," indicates a plurality of beings; unless indeed we are to suppose, that God is conversing with his own virtues, which he employed as instruments, as it were, to create the universe and all that is in it...But all these things are similitudes, and forms, and images, among men; but among the gods they are prototypes, models, indications, and more manifest examples of things which are somewhat obscure; but the unborn and uncreated Father joins himself to no one, except with the intention of extending the honour of his virtues.
Now Philo was certainly reading into the text, but he did it through the lens of Hellenized Judaism. (In reality the 'Like one of us' was a reference to the divine council of gods, but that is another topic.)
As former JWs it is difficult to accept that we had overlooked the many examples in the OT that personified the Dibbur/Word/Logos or the Kabod/Glory of God or the Angel of Yahweh etc. but we did. Interestingly after the Christian expansion of these concepts through the embodiment of Jesus, the Rabbis generally condemned the notion of emanations (second power theology) as too polytheistic sounding, though rarely the concept reappears in the Talmud, mainly as the Memra/Word.
So no, the Jews did not have the Trinity doctrine, yet the Trinity doctrine was in fact an attempt to explain many aspects of Jewish writings.