Actually, according to Jewish Law, the answers given are mostly false. If you have Jewish blood, then you definitely have Jewish ancestry, but it doesn't make you a Jew. You are a Jew if your mother's mother's mother's mother's mother (etc) was Jewish. That is the only way (unless you convert according to Jewish Law.) Also, if it does so happen that you are Jewish according to Jewish Law, then you are a Jew in every sense of the word and can become baal teshuva (repent and return to Torah-observant Judaism) and receive any rights as a Jew. As a Christian though (not trying to offend anyway, just telling you how it works), you would be considered a Jew who is an apostate and are not eligible for aliyah under the "Law of Return", nor could you get other Jewish rights such as being buried in a Jewish cemetary, for example. In otherwords, if your ancestry is as I said above, then you are a Jew, but if you are a practicing Christian (or a Messianic), then you are considered an apostate.....
G~d bless,
~Little_Lily613~