The written תּוֹרָה / Torah (instructions / teachings) is "G-d's little instruction book" on how to live a G-dly life, grow spiritually and become holier. G-d gave seven instructions to all of mankind (these are called the Noahide mitzvot because they were given prior to G-d's contract with Abraham). At Sinai the Jewish nation agreed to an additional 606 מִצְווֹת / mitzvot. The Talmud tells us (Tractate Makkot 23b) that there are 613 commandments in the Torah; 248 Positive Commandments (do's) and 365 Negative Commandments (do not's) to be found within.
Keep in mind that no one has the right to change any of the mitzvot. "Do not add to the word that I am commanding you, and do not subtract from it. You must keep all the מִצְווֹת / mitzvot / commandments of G-d your L-rd, which I am instructing you." D'varim / Deuteronomy 4:2.
Note the wording, the Torah says "do not add to the word that I am commanding you."
The mistake of many missionaries is that they read the "do not add. . . and do not subtract" and think that this means everything ever -- yet the Torah itself says no such thing. The Torah clearly says "to the word that I am commanding you." The commandments (mitzvot) cannot be changed PERIOD.
This does not mean that we cannot add things outside of those commanded by G-d. Indeed there is a category called מצוות דרבנן / mitzvot d'rabbanan (rabbinical commandments). There are שבע מצוות דרבנן / seven mitzvot d'rabbanan. Along with Chanukah and the Shabbat candles you can the holiday of Purim, praying three times a day, reciting kaddish after the dead, making a blessing before eating, and washing of the hands (which the Christian bible gives as a requirement in it -- before it ever was one -- yet more proof that whoever wrote it did so long "after the fact").
Another aspect of rabbinical mitzvot are rabbinical "fences" meant to prevent people from accidentally violating one of the Torah mitzvot. For example the Torah tells us to keep dairy and red meat separpate, but the rabbis extended that "fence" to include poultry. How is this not "adding" to the original mitzvah? It doesn't change it (you still cannot eat dairy with red meat) -- it merely adds other kinds of meat so that people won't accidentally eat dairy and red meat because they are used to doing so with poultry.
the Torah commands us to follow rabbinic laws, so there’s nothing “only” about it!
"you must keep the Torah as they (the judges) interpret it for you, and follow the laws that they legislate for you. Do not stray to the right or left from the word that they declare to you." D'varim / Deuteronomy 17:11.
Ergo the judges have the authority to create new legislation (such as not eating poultry with dairy) and this is not changing the previous mitzvah (do not eat red meat with dairy) because they did not suddenly permit eating red meat with dairy -- they did not change the mitzvah). . . Make sense? They simply added a "fence" that would help people avoid accidentally eating red meat with milk. . .
The origin of the בית דין הגדול / Beit Din HaGadol (the Great Court) Sanhedrin can be found in the Council of the seventy elders founded by Moshe Rabbenu (Moses): "G-d said to Moses, 'Assemble seventy of Israel's elders - the ones you know to be the people's elders and leaders." (Bamidbar / Numbers 11:16).
Since the time of Moses we Jews have always had judges / courts of law / democratic governing bodies ever since. Even in the days of King Saul and King David this was true. Berachot 4a implies that the אב בית דין / Av Beit Din (Head of the Court) of the "supreme court / congress" (Sanhedrin) during David's reign was בניהו בן יהוידע / Binayahu ben Yehoyada. David was likely the Nasi Beit Din (Prince / President of the Court). Meod Katon 26a says that King Saul was president of the Sanhedrin in his reign, with his son, Yonatan, also a member.
Want more? Read about Jehoshaphat (Y'hoshafat): "in Jerusalem, Jehoshaphat set up judges of the Levites and the priests and of the chiefs of the fathers' [houses] of Israel, for the judgment of the L-rd and for quarrels, and they returned to Jerusalem." (Divrei Hayamim II / 2 Chronicles 19:8).
Jewish courts also determined who was a prophet (Jesus was not a prophet). Jewish courts decided whatt books went into the bible and which were left out (אַנְשֵׁי כְּנֶסֶת הַגְּדוֹלָה / The Men of the Great Assembly).
Jewish courts do not use juries. The smallest Jewish court consists of three judges, and these courts still exist today. In ancient times appeals could be made to courts with 23 judges, all the way "up" to the great court which had 70 judges (this number is fixed in the Torah).
A three judge court could, for example, rule on theft cases. A capital punishment case (for example) required a court of 23 judges (a small Sanhedrin), but false prophets had to be adjudicated by the great Sanhedrin of 71 judges (the Great Sahnedrin). (Mishna, Sanhedrin 1:1-6).
It is important to realize that missionaries who reject Jewish judges claiming they rely "only" on the bible are actually rejecting the Jewish bible since the bible itself dictates listening to Jewish judges -- and it was Jewish judges who decided which books to include and exclude in the T'nach (e.g. Prophets and Writings).
What type of questions might come before a court? Adjudicating issues around the 613 mitzvot. "what if the person who stole something was starving and he stole food from a market to avoid death?" That is a particular case that might come before the court that would require them to adjudicate in that specific instance.
Jewish law has many safeguards in place to prevent courts from "running amok." This is one reason the death penalty was so rare (more than one in 70 years was rare). A capital (death) case required 23 judges. No Jewish court can have less than three members. Judges must be expert in the Torah. Majority rules -- but if the court is unanimous for guilt in the case of a death penalty case the person is not put to death. The rules for passing a sentence of death were nearly impossible to meet (the two eye witnesses, a majority of the judges passing the sentence, but at least two on the side of innocence, etc.) that very few were ever meted out. . .
No doubt there are some flaws -- not within the laws (those come from G-d) -- but in human implementation of the law. Still the safeguards are remarkable. The written Torah tells us:
To appoint judges and officers in every community of Israel (D'varim (Deuteronomy) 16:18)
Judge honestly between each man and his brother (D'varim (Deuteronomy) 1:16)
Do not testify as a false witness against your neighbor. (Sh'mot (Exodus) 20:13)
A single eyewitness may not testify against a person where the death penalty is involved. (Bambidar (Numbers) 35:30)
Do not pervert justice. (Sh'mot (Exodus) 23:6)
Do not join forces with a wicked person to be a corrupt witness. (Sh'mot (Exodus) 23:1)
Do not follow the majority to do evil. (Sh'mot (Exodus) 23:2)
Do not speak up in a trial to pervert justice. A case must be decided on the basis of the majority. (Sh'mot (Exodus) 23:2)
Keep away from anything false. (Sh'mot (Exodus) 23:7)
Do not accept bribery. Bribery blinds the clear-sighted and twists the words of the just. (Sh'mot (Exodus) 23:8)
You must investigate and probe, making careful inquiry. (D'varim (Deuteronomy) 13:15)
One witness must not testify against a person to inflict any punishment or penalty for a crime that he may have committed. A case must be established through the testimony of [at least] two or three witnesses. (D'varim (Deuteronomy) 19:15)
This is what you must do] if a corrupt witness acts to testify falsely against a person. Two men who have testimony to refute [the false witnesses] shall stand before G-d, before the priests and judges who are involved in that case. The judges shall carefully interrogate [the refuting witnesses], and if the [first] two witnesses are found to have testified falsely against their brother you must do the same to them as they plotted to do to their brother, thus removing evil from your midst. (D'varim (Deuteronomy) 19:15-19)
Do not give anyone special consideration when rendering judgment. Listen to the great and small alike, and do not be impressed by any man, since judgment belongs to G-d. If any case is too difficult, bring it to me, and I will hear it.' (D'varim (Deuteronomy ) 1:17)
Do not pervert justice (D'varim (Deuteronomy) 24:17)
The last few posts have discussed what the oral Torah is and how it came to be written down over hundreds and hundreds of years -- first as the מִשְׁנָה / Mishna (study by repeating -- from the word שנה, or “to review”) and then גמרא / Gemara (study). Together they are called the תַּלְמוּד / Talmud which means “instruction." It is important to understand that the commandments are given in the Torah -- nothing in the rest of the bible (Prophets / Writings) has the authority to change the mitzvot.
The Talmud is a combination of explaining "how to" fulfill the written mitzvot (for example -- the written Torah may say to put a sign upon your doorpost, but it does not explain what that sign should be, or where it should be placed. Those explanations are found in the Talmud). Debate around minor points are discussed at length in the Gemara (along with stories, humor, etc.).
G-d gave our courts the authority to make legal rulings based on the Torah. In that sense they cannot be wrong -- if they follow the rules and do so attempting to do the right thing then they ARE doing the right thing even if the ruling itself seems wrong (or is even reversed at some later point). Think of the United States of America as an example. There were times when women could not vote. There were times when blacks were slaves. At the time, ruling within the laws of the nation, the United States Supreme Court made rulings that were considered legal and binding and "right." Over time the laws changed -- women voted, blacks were free men and over time we would like to think we are being truer to the freedoms inherent in our country.
The Jews have always had halacha (Jewish law) which comes from G-d. That does not change and cannot be changed. It DOES have to be interpreted -- to specific legal cases and new occurrences (when would a Jewish astronaut observe "sundown" for Shabbat in outer space?).
One thing that is difficult for non-Jews to grasp is that we are co-creators with G-d. He put us on this planet to learn and grow and to make decisions. This makes the question of "right or wrong" decisions somewhat academic -- we are SUPPOSED to do this -- and that is why G-d commanded us to create courts and judges and to listen to them.
The key for understanding Jewish courts and judges is to realize that when our Sages, Rabbis, and the Sanhedrin teach something, it must be consistent with the contents of the Torah. If it is consistent, then it's a true teaching; if it is inconsistent, then it is a false teaching, and it is to be discarded.
Some non-Jews seem to think that what is in the bible is ancient and does not apply to today. Jews know that the Torah does apply to today and we live our lives by it. There are still Jewish judges and courts of law, as there have been in every generation since Moses himself. The Sanhedrin was the supreme council of Israel. As long as it stood, it was the supreme court and legislative body in all matters of Torah law. As such, the Sanhedrin was entrusted with keeping and interpreting the Oral Torah.
The Great Sanhedrin (the Supreme Court / Legislation) met in the Temple area, in the Chamber of Hewn Stones (לשכת הגזית / Lishkat HaGazit). It was only in this chamber that the Sanhedrin could perform all its functions, including the trial of capital offenses. In 28 CE (3788 on the Jewish calendar) the Chamber was destroyed, thus eliminating the Sanhedrin's ability to pass the death penalty (ergo the "trial" of Jesus as described in the Christian bible could never have happened).
The Sanhedrin itself moved to another room on the Temple Mount initially, and when Jerusalem was destroyed in 68 CE (3828) it moved to the city of Yavne. For the next century it moved from Yavneh to Usha, then to various other locations. The Sanhedrin remained in existence until the 4th century of the common era when it was eventually disbanded. Today there is no great court, but there are many three judge courts throughout the world. These courts ordain rabbis, perform conversions and oversee other aspects of daily Jewish law.