“You shall not add to the word that I command you, nor shall you subtract from it, to observe the commandments of HaShem, your G-d, that I command you.” D’varim / Deuteronomy 4:2.
What does that mean?
Some missionaries seem to think it means that if something isn’t mentioned in the Torah it is forbidden. But that doesn’t make sense – after all the Torah tells us "And you shall observe all that they shall instruct you" (D’varim / Deuteronomy 17:10).
Observe all that they instruct you (who is “they”?) – but don’t add to the word or subtract from it. . .
If we could understand everything Sola Scriptura (by reading the bible) there would not be the command to listen to our instructors and observe all they tell us to do.
There are 613 mitzvot in the Torah for Jews (and 7 for non-Jews). When D’varim (Deuteronomy) tells us not to add or subtract from those mitzvot it means DO NOT CHANGE THEM.
If the Torah tells you “do not steal” then do not steal.
It doesn’t mean “you can steal a colt and donkey” (Matthew 21:7) but don’t steal anything else. Or “if you think you are the messiah you can steal stuff.” No, “do not steal” means DO NOT STEAL.
Another example. The Torah tells us it is a mitzvah to marry. “To marry a wife by means of ketubah and kiddushin.” D’varim / Deuteronomy 22:13. Ergo one should marry. (Jesus failed to fulfill this mitzvah).
Does it say “marry only one woman”? No it does not. It simply says to marry. The Torah is silent on whether marriage should be monogamous or polygamous – so neither one is commanded. That choice is left open, one is not adding to or subtracting from the mitzvah to marriage by marrying one person or more than one – the Torah is silent on the subject.
However, since the Torah also tells us to listen to our judges and their instructions they may build a fence around a mitzvah to protect it. Again, if the mitzvah is silent on an aspect this is not adding to or subtracting from the mitzvah – as the mitzvah itself has not been changed.
Read Sh’mot (Exodus) chapter 18: “But you must [also] seek out from among all the people capable, G-d-fearing men - men of truth, who hate injustice. You must then appoint them over [the people] as leaders of thousands, leaders of hundreds, leaders of fifties, and leaders of tens. 'Let them administer justice for the people on a regular basis. Of course, they will have to bring every major case to you, but they can judge the minor cases by themselves. They will then share the burden, making things easier for you. 18:23 If you agree to this, and G-d concurs, you will be able to survive. This entire nation will then also be able to attain its goal of peace.'” Sh’mot / Exodus 18:21-23.
Now we know who the "they" is -- the judges. The learned men from all of the tribes who instruct us and interpret the mitzvot to new situations and problems. Another word for judge is "rabbi." Rabbis sit in Beit Din (courts of law) and adjudicate cases (including conversions) even today. The higher courts (the 23 judge and 70 judge Sanhedrins) do not currently exist, but will in the messianic era.
From the time of Moses to today there have been Rabbis (teachers / judges) from all the tribes who teach and mete out justice. They apply the mitzvot to various legal problems (this is what much of the Talmud is doing – describing the rules in a given situation). . . and it is ALL biblical. Rabbinical courts do not “change the law” the rabbis are doing exactly what G-d instructed them to do – follow the rules and apply them using the Torah as their guide. (The word "rabbi" / רב (rav) is derived from the word רַבִּי rabi (my master).
While we are forbidden from changing the 613 mitzvot in the Torah, there is nothing wrong with adding a new rule (such as the observance of Purim or Chanukah). We simply are forbidden from changing the 613!
Consider the weekly blessing over the Shabbat candles. This is not a mitzvah given in the Torah -- instead it is a mitzvot d'rabbanan (rabbinical instruction). Other rabbinical instructions include celebrating Chanukah and the holiday of Purim, along with praying three times a day, reciting kaddish after the dead, making a blessing before eating, and washing of the hands. . . none are in the written Torah, but the fact that we do them is not “adding to or subtracting from” the 613 mitzvot. The 613 mitzvot have not been changed!
One reason there is an oral "how to" perform the mitzvot is because people are not blocks of stone -- never moving or changing. Life is constantly evolving and with new conditions must be someone who can interpret and safeguard His mitzvot and legislate Jewish life. G-d imparted the authority to his judges -- and this is in the WRITTEN Torah.
Adding to or subtracting from the Torah mitzvot would be having the written Torah say "Shabbat is on Saturday" (the 7th day, Sh'mot / Exodus 20:10) and having someone come along and say "let's make Shabbat Sunday."
Torah tells us (for example) to observe the Shabbat and keep it holy. We are told "no melachot" (what to do and not do -- like light a fire is a no-no). The Rabbis said "let's have everyone light candles just before sundown and say a "thank you to G-d" and that will bring the family together and make everyone realize that Shabbat is beginning. It will 'set the tone.' Now how exactly does that CHANGE what G-d told us for Shabbat?
Every one of the mitzvot d'rabbanan begins with the blessing "Blessed are you HaShem our G-d who has commanded. . ." The lighting of the Shabbat candles, Purim, Chanukah, etc. av Nehemiah says: From 'Ask your father and he will tell you, your grand-parents, and they will say to you' (D'varim / Deuteronomy 32:7)'" (Babylonian Talmud, Shabbat 23a).
This authority comes straight from the Torah: “You shall act according to the law they shall teach you and according to the judgment that they shall tell you; do not deviate from the judgment that they announce to you either to the right or to the left." (D’varim / Deuteronomy 17:11).
And just who are the “judges” who have the training, the knowledge, the experience and the authority to decide matters of Torah-law? The Rabbis (who are both instructors and judges).
The Being Jewish website has excellent articles entitled Haven't Rabbis Changed the Laws, Why Can't Jewish Law be Altered and How Do We Know Our Tradition is Correct.