Shalom and thank you for your question! You say that your great-grandmother was Jewish, and that (although you were presumably brought up as a Catholic,) you have become ‘disenchanted’ with Catholicism, and wish to know if you can still claim your ancestral Judaism.
The answer is yes, provided your lineage is on the maternal side. In other words, if your mother’s mother’s mother is the great-grandmother in question, you are Jewish regardless of any other beliefs you were taught or trained in, or even chose of your own accord. In such a case no changes need be made at all. If the great-grandmother in question is from your father’s side, or your mother’s father was her son but your mother’s mother was not Jewish, then you can become a Jew by a conversion process carried out according to Jewish law. This is a process which needs guidance and it is the duty of the Rabbis involved to try to deter you at first, because Judaism requires great commitment. The Jewish people have a history of suffering from constant persecution, having had to flee from country to country in the exile after being exiled from the Holy Land with the destruction of the Second Temple. In the Haggadah of Passover it says “In every generation they have stood upon you to destroy you, but the Holy One Blessed be He saves you…”
The Torah teaches that a code of belief and behavior was taught to Noah, in the form of seven laws, called the Noahide laws. This happened after the great flood in the days of Noah. G-d made this covenant with Noah, and all humankind today is descended from him. These laws include prohibitions against stealing, killing, committing adultery or other acts of sexual immorality, eating flesh torn from a living animal, and the obligation to set up courts of justice. There are many details, and today there are places where non-Jews study these laws and practice them. It is desirable that people who are not Jewish avail themselves of the opportunity to learn these laws and practice them, because they are the basis of just and moral society.
There are authorities that say that after the giving of the Torah, the Jewish people were no longer considered children of Noah, but in the Talmud it is explained that the obligations that Jews have actually include the seven Noahide laws.
Going back to the topic of your lineage, it may require tracing your family genealogy in order to verify what you were told.
Again, if you find that the lineage is matriarchal, you are a fully- fledged Jew.
If this is the case, then you can find out more about how to practice your Judaism from a local Chabad center, they will be very happy to help you.
Judaism is based on six hundred and thirteen laws which have their sources in various places in the Five Books of Moses. Maimonides, (also known as the Rambam,) is a classic codifier of Jewish law, and he compiled important classic works explaining them. He also wrote a widely accepted list of what those laws are, called Sefer HaMitzvot.
The laws themselves and the details of how to practice them are discussed in the Talmud, and presented as final legal decisions in the volumes of the Shulchan Aruch. All of this is called the Oral Law. In each generation new books are written, explaining how to apply the law in the specific circumstances of that generation. This makes for a very extensive library of Jewish law, which is both fascinating and intricate.
