At the time Jesus lived, prior to the destruction of the second Temple, there were various sects within Judaism. Some say that there could have been as many as several dozen. One of these was a sect led by Jesus.
He was rejected by the Jews of his time, a generation which included some of our greatest Rabbis. The Christian response to the rejection of Jesus by his own people has been nearly two thousand years of efforts to convert Jews to their beliefs. When a Jew accepts Jesus he not only rejects the history of his people but, by adopting Christian faith he places himself outside the Jewish religion.
Christians believe in the Trinity, that G-d consists of the Father, the Son and the Holy Ghost. They say that this three-part G-d is the same as the G-d worshipped by the Jews.
The Torah says, "Hear O Israel, the L-rd is our G-d the L-rd is One." This we say in the Shema, we have it on our doors in the mezuzah and on our hearts and minds in the tefilin. Christians believe that one cannot approach G-d except through Jesus. All prayers must therefore, be in his name. This is called Mediation.
The Torah starts the Ten Commandments with, "I am the L-rd your G-d, who brought you out of the land of Egypt, from the house of slavery. You shall have no other gods before me". Simply G-d does not want us to have any other deities even if we still believe in Him. Limiting an all knowing G-d by claiming He needs a mediator is a violation of this commandment.
Christians say that Jesus was a Prophet who came to change the way it used to be. The Torah warns us about this, "If there arise among you a prophet or a dreamer and he gives you a sign or a miracle. And the sign or miracle comes to pass and he calls you saying 'Let us go after other gods, whom you have not known and let us worship them.' You shall not listen to that prophet or dreamer. For G-d is testing you, to see whether you love the L-rd your G-d with all your heart and with all your soul." (Deut. 13:2)
So are you a better Jew, a fulfilled Jew by accepting Jesus? No. By accepting a Christian god you commit a cardinal sin. You become a traitor to your people and cannot be counted a Jew.
The Torah teaches, "It is enough that you carefully observe everything
that I am prescribing to you. Do not add to it and do not subtract from
it."