If a Conservative, Reform or Reconstructionist Jew asks an Orthodox Jew, "What is the difference between Conservative, Reform or Reconstructionist and Orthodox Judaism? Who says that Orthodox Judaism is the correct one to follow?"
How should an Orthodox Jew politely answer without making the person feel like two cents or making a chillul Hashem?
When I was shopping for a synagogue and a rabbi I was pretty methodical about it. I didn't want to judge just based on what I'd heard people say about different communities. After I'd visited a bunch and started to narrow things down, I met individually with local rabbis from the Reform, Conservative, and Orthodox movements. (We didn't have any local Reconstructionist rabbis.) Among my other questions I asked each of them: What makes your movement different from the others? Why is yours the correct one?
This was rather a while ago, but fortunately I took notes. The Orthodox rabbi I met with, who was quite well-regarded in the community, told me the following:
Orthodoxy holds that the torah is the word of God. (Well duh, I said -- who doesn't?) Reform, he said, holds that the torah was written by men. He said that Conservative does too, though this was later disputed.
Observance is a journey and a goal, not all-or-nothing. There are pious and less-pious Jews (in all movements), and just because you can't do 100% doesn't mean you can't do anything. Choose a movement based on what you believe, not what you are able to practice today. You're welcome in Orthodoxy even if you aren't doing everything you should yet.
Orthodox Judaism makes demands of you, sometimes uncomfortable ones. Other movements are less demanding; if you're looking for a community that will make you feel good rather than one that will make you work, Reform may be a better fit for you.
Based in part on that experience, I think the key points to cover would be:
I once was asked to speak to a conservative bar/bat mitzvah class to introduce Orthodox Judaism. The other 3 rabbis had already spoken in previous meetings. Shavuot was approaching. After I finished, the Chazan (conservative) got up and made a fascinating comment, before opening the floor to Q&A. "isn't it interesting that the Conservative, Reform and Reconstructionist rabbis spoke about what Mitzvot they do [and/or don't do] yet the Orthodox Rabbi did not mention that at all. Why Rabbi?" I answered The main issue of Orthodox Judaism is the belief that we have been handed down, from generation to generation, father to son and rebbe to student, the Torah and the Mesorah (traditional explanations). Once we have a firm belief in believing that the Torah and Mesorah are the word of haShem, THEN we do our best to keep the words of haShem. Another time, asked to speak to the parent body of a nondenominational school, I shared a point that I heard from one of my Rebbeim, Rabbi Yisroel Miller, currently of Edmonton, Canada."After 120 years, I can hope to have one solid 'excuse' for not excelling in keeping all the Mitzvot. "If I would have had more time, I would have made it! It's not my fault that the generation of the flood caused our days to be shortened!" "And that will work," said Rabbi Miller. "But that excuse will only work if haShem pulls out 'His celestial computer' and computes that you would actually have made it and not gone deeper into the morass!!" At that point, I stopped and posed the following question: "which is better: to accept 50% of the Torah/Mitzvot and keep all 50% or to accept 100% of the Torah/Mitzvot and aim for 100% and keep only 10%? Everyone agreed 10% of 100% is better than the entire 50% and no more.
As a person in the process of conversion, I am also "shopping" different movements. The interesting thing here where I live is that the local board of rabbis, which includes every movement, asks potential conversion candidates to attend an 18 week introduction to Judaism course. During this series of lectures, rabbis from each of the movements will teach a class or two, and the classes are spread around to different synagogues.
So far, I can tell you that the young, Reconstructionist rabbi ( early 30s) seemed very influenced by 19th century higher criticism, and as such took a critical view of Torah, miracles, and Jewish history. He was also disparaging of "converts", believing that Judaism is an ethnic and cultural religion only. His lecture spawned many calls to the Board who then apologized for his message to some extent and assured the class that his views are not shared by the majority of Jewish thought.
The Reform rabbi ( in his 60s) was more spiritual, spoke at length about God's role in the history of Israel, believed in the Torah and the Exodus and spoke highly of the Talmud. I was expecting a more liberal theological approach, frankly, and was surprised by how "biblical" he was.Pleasant experience over all.
The Conservative rabbi ( late 30s) is very Tanakh-centered, almost Orthodox in his teachings and observances, very spiritual, yet kind and friendly.I think some would call him "Conservadox", as he is by far the most "frum" Conservative rabbi I have met yet. Very impressed with him so far.
I will post back after the next few rabbis lecture, including the Orthodox one, who will be handling the topics of kosher and halacha. But one thing is for certain, there are many different views and approaches within Judaism!
You phrased your question very specifically in that you said "what do you do when someone from the conservative reform movement asks you..." as opposed to someone who is completely unfamiliar with these movements, or someone who is familiar or unaffiliated. Therefore i think your answer should be different and not about "educating" or "explaining" the differences, since it's likely that the person asking you might already know some of these differences. Therefore my answer is as follows:
You should answer why YOU believe in Orthodox Judaism, and answer it focusing it on you, not drawing lines around the other movements. For example, rather than say "The reform movement heretically believes that man wrote the Torah," your answer should be "It's important to me that the Torah was divinely given by God to Moses. If i were to believe that it was written by man then i might find myself nonreligious, or at risk of picking which miswoth/miztvot i wanted to keep and which ones i wanted to discard; which would stop my spiritual and moral growth." In the latter answer, you have answered their question by stating the opinion of Orthodoxy that the Torah is God given, explained why it's important to you, and given an open door for the person you are speaking with to empathize with you, understand where you are at, and respect you for it. You have also managed to not insult anyone nor speak lashon hara, nor committed hillul hashem.
"It's important to me... If i were to believe that it was written by man... which would stop my spiritual and moral growth." Your example answer would be an honest response from someone who makes himself "believe" in Orthodox Judaism for some functional purpose rather than because he really thinks it is true. It doesn't seem very suitable for someone who believes in Orthodox Judaism because he actually thinks its tenets are true.