The God of the old Testament , or the Tanakh if you prefer, and his expectations from his worshipers is quite different from the message that Jesus taught. So much so that it might as well be a different God, completely different approach on how to live and how to worship God. To put it as simply as possible Judaism is very outer in it's orientation while Christianity in it's essence is very inner. If you believe it's the same God and that God us unchanging as is commonly taught than you might as well follow all of the 613 mitzvot, all the laws proscribed by Yahweh, rather than pick and choose because some are inconvenient. Frankly it deserves a a second look for those who claim to be Christians, the relationship between Christians and Jews is a long and complicated one and your not going to get it with out some very deep diving. Like an archeological dig, there are many layers to it, and you have to often examine apocryphal and gnostics writings to try and uncover what was really going on around the time period of the second temple. The Babylonian captivity changed Judaism forever and some Jews came back with very different teachings, including those regarding the afterlife, which did not exist in pre exile Judaism. Sometime after the return of the exile is when Christianity was born, the book of Daniel is when we start seeing the seeds of the Christian movement.