Christians are a Jewish sect. I know this annoys the more racist Orthodox and especially Chasidics, but that's just a fact.
Don't you think that Jesus deification contradicts the core principle of Judaism about oneness of God?
No. We see references of 'God' in the plural sense beginning in Genesis with the creation narrative. The Old Testament has many 'voices' throughout its writings. The Christian 'Trinitarian' doctrine doesn't violate any ' Oneness' doctrine; that's a red herring argument from some Orthodox rabbis desperate to bring many Jews back into the fold after they did their best to alienate and run off as many of their own people for some 500 years; with the advent of such sects as the Essenes, Christians, Pharisees, the popularity of Greek culture and writings with Jewish scholars, the various 'schools' that all started popping up around 200 B.C. or so, the status quo was threatened from a number of sides, not just Da Evul Xians; the latter was just far more successful and closest to the original dynamism of the Mosaic era, and for very good, and obvious, reasons it took over and drove progress. The NT is not separate from the OT, historically, literally, or philosophically, despite all the confusion to the contrary. It is reliant on the Torah, as was Jesus ministry.
Jewish prophets talked about the Messiah who would be a descendant of David. It was nothing said about God becoming a flesh. Right?
What do you think about Sabellianism?
According to both Mathew and Luke, Jesus was descended from David. Not sure about your second point, 'becoming flesh', since I don't have much interest in analyzing and doing extremely detailed exegeses on every single rhetorical and literary reference in the entire book, but when I get some time in the next few weeks I will see if that is indeed the case. As for 'Messiahs', they came in several forms; some considered Cyrus to be a 'Messiah', so the term isn't as exclusive as some might think. We also know prophesies didn't necessarily all come true, for an assortment of reasons.
Sabellianism doesn't really fit the trinitarian doctrine; just speculative sophistry, but not meaning to be deceiving or fake, as as most Gnostic rubbish is. I leave it to the beleivers to sort out their own issues re that; I'm happy with the trinitarian explanations. Mathew 28:19 in the Textus Recepticus is more than good enough for me. I can see where there will be arguments over such stuff among some, but since I'm not a believer those types of unnecessary semantic arguments don't concern me. I'm more a fan of Christianity's much more dynamic and positive influences on western history its culture of progress, and it's suppression of mindless paganism.