>>810057
It does has Jewish roots, but it's not a form of Judaism. Read Acts of the Apostles for more info, or the Epistle to the Romans, and it'll make it clearer. Christian Jews continued with their Jewish beliefs, as well as their new Christian ones. Pagans that converted to Christianity did not take part in those rituals. Faith in Christ is enough to be a Christian and Paul says that if rules bring you closer to God, that's great, but implies that following Christ through faith alone is better (Romans 14). Of course, proper faith is expressed by acts, it's just that following rules and following Christ are different things.
>What is it that makes Christianity not Jewish?
Basically, Christianity does come from ancient Judaism. The term "ancient" is important here. When Christ came, some Jews accepted him, some rejected him. The first ones became Christian and eventually ceased to be Jewish, and the second ones became the only kind of Jews. Those who rejected Christ, who killed him and chose Barabbas instead, did so cause they wanted an aggressive Messiah, someone to lead them against the Romans (the external enemy), instead of someone who carried a message of love and self improvement (to battle the internal enemy). These Jews latter rose up against the Romans, and it ended up with the destruction of the Temple of Jerusalem, just like Christ said (Mathew 24:1-2), in the year 70 AD.
Why? They rejected Christianity and forty years later the Temple, the way Jews could communicate with God through the covenant, is destroyed. When you read the Bible, it becomes clear: the Old Testament is the instructions on how to create a perfectly ordered society, how to build civilization in the middle of nature. Once society is stable, once Rome dominates all the western world and is politically stable, Jesus comes. The New Testament shows you the way to personal development, to build the perfect man. That's what Jesus is: by being both man and God, He is human perfection. Now that we have His example, we can aim to imitate it.
THAT is what makes Christianity not only not Jewish, but a polar opposite. Judaism is for creating a society in the middle of the desert and Christianity is for personal union with God. Jews reject that. Jews are always trying to build a Jewish society, to mold existing ones to their desires and needs. That's also why so many Jews have embraced ideologies that dismiss personal responsibility, blame everything on society and aim to change it radically to create a new perfect society in it's place - that's why so many Jews have been Marxists.
Now, let's briefly address one thing: "What is it that DOES make Christianity Jewish?"
Some branches of Christianity are arguably Jewish. I know this may rub the wrong feathers, but without wanting to be provocative, I have to say that some protestant churches are more Jewish than Christian. With their "free interpretation of the Bible" they give a lot of importance to the Old Testament and read the New one under the light of the Old (it should be done the other way around). While trying to show everyone how non-catholic they were, they had to adopt new theology, in many cases copy-pasted from Talmudic tradition (the Talmud being a compilation of Jewish books written since the destruction of the Temple), in other cases copy-pasted from past heresies (which, in turn, often had theology influenced by Jewish thought). It seems to me this Judaic Christianity is specially prevalent in Puritan and Calvinist areas, like the Netherlands and the USA. This is just an outsider's impression, so I could be wrong.
I think that's about it. Sorry if the post is too long, but I hope it'll answer to your question.
TL;DR: Ancient Judaism was split into Christianity and modern Judaism when Christ came. Christianity does have roots in ancient Judaism, but it's completely different than (and opposite to) modern Judaism. Why? Read the whole post for that.