Your post is precisely the reason that Jews do not accept the Christian message. The primary reason they don't accept it is that it is unnecessary. Let's forget the fact that Jesus isn't the Messiah (it is this fact that makes Christianity false). If we look just at the message of Christianity, we can see that it is unnecessary from the perspective of the Jewish tradition.
The Christians will say:
1. Mankind is a sinful being and because of sin he is separated from God--Judaism, for the most part, would agree with this.
And that's about all they agree on. Is that man is sinful. Why he is sinful, the rectification of sin, what should be done with those who are sinful, all of that is disputed. Christianity will come to believer in the Tanakh and say "You cannot atone for your sins before God. You, by yourself, are unable to go to God and be forgiven of your sins. Jesus' sacrifice is necessary for you to receive salvation."
Fact of the matter is that such statements are baloney. God Himself says over and over and over and over again throughout all of Tanakh that a person should and can repent. He says that we are able to keep the law that He has given to us. I hear Christians say a lot "Jesus fixed the problem between man and God." When I ask what that problem was, they usually reply, "The fact that mankind is separated from God through sin."
My response is always, "What's wrong with the methods described in the Tanakh (Charity, Repentance, and Sacrifice?)?" They will usually say, "That was inadequate to serve as a method of forgiveness." When I ask them the following question, they are unable to answer and I have yet to me a Christian with an answer. Where in the Bible does it say that mankind is unable to attain "salvation" through the methods given in the Tanakh? Where does God say that we cannot repent WITHOUT sacrifice. Christians try to say "the Bible says 'there is no forgiveness without the shedding of blood'". Little do they know, that verse comes from the New Testament. God Himself says no such thing.