Is it okay for one to call a rabbi and Torah scholar or teacher by his first name? I heard that in Lubavitch we are not particular in this matter and call even great rabbis and Torah scholars by their first name and even a nickname.

From a pure letter of the law halachic perspective, it is only prohibited to call someone by their name without an honorary title, if that individual is one’s main Rebbe in Torah study from which he has learned majority of his Torah knowledge. [By such a person, one who calls the individual by his name without an honorary title, is defined in the Talmud as an Apikores.] Nevertheless, it is proper for one to be stringent to preface the person’s name with an honorary title, such as rabbi, even if one only learned minority of one’s Torah knowledge from him. Furthermore, it is proper to preface an honorary title to the person’s name, even if he is not one’s personal rabbi from who he has learned Torah, but just a general Torah scholar.

Now for the custom in Lubavitch: The Rebbe once stated that the custom in Lubavitch is not to be particular in mentioning honorary titles when discussing an individual, and on the contrary the greater the Chassid was, the less honorary titles he was given. For example, the Radatz, who was one of the greatest Chassidim and rabbis and Torah scholars of the generation [being one of the few that received Semicha from the Rebbe Maharash and a special blessing from the Tzemach Tzedek], I [the Rebbe] recall from my youth that he was referred to without any honorary titles. People didn’t even refer to him as Rav Dovid Tzevi Hirsh, but rather simply as Dovid Hirsh, and at times not even as Dovid Hirsh, but Dovid Hershel. The Rebbe then continues and says: I searched to find the source for this behavior in Jewish law and I found that the matter is explicit in the Talmud regarding the rebellion of Uriah who referred to Yoav as his master in front of Dovid, and so too by Chassidim the image of the Rebbe is always in front of their eyes, and when in front of their Rebbe they cannot call someone else by an honorary title or master or teacher.

However, in my opinion, especially in today’s times, this previous custom in Lubavitch should no longer be practiced, as the above applied in a time that people had deep respect and revere for Rabbanim and Torah scholars, and the use of a nickname or first name was not meant in any way as a form of denigration or belittlement. Everyone remembered who were the scholars and leaders and Chassidim closest to the Rebbe from home they should learn and received tutelage in both Torah knowledge and the ways of Chassidus. However, in today’s times, our generation suffers from a denigration and belittlement of Torah scholars and righteous individuals, to the point, that the young believe they know better than the old, and the opinions of the elderly rabbis and scholars often mean nothing to them. This itself comes from a true belief that there is nothing special about a Torah scholar or vintage Chassid, and Ani Viefsi Od. Accordingly, in order not to strengthen this great decline within the morale of communities towards their rabbis and Torah scholars therefore, specifically today there is no room to act as above. It’s important also to note that despite the above talk of the Rebbe, the Rebbe spoke and acted with utmost respect to other individuals even in the lifetime of the Rebbe Rayatz, as is evident from the many honorary titles which preface the name of an individual throughout the letters of Igros Kodesh. Umaaseh Rav! To note, that a precedence for this concept of changing the custom in order to battle a belittlement of Rabbanim, was publicly done by the Rebbe in 1986 in which he stated that despite the fact that the Chabad custom is not necessarily to give the Aliyah of Aseres Hadibros to the Rav on Shavuos, this year they are to do so due to the belittlement in the position of the Rav.

Sources: See Michaber Y.D. 242:15 and 30; Sanhedrin 80a; Tumim C.M. 15:1; Leket Hakemeach Hilchos Kevod Chachamim; Tzelach Brachos 4a; Tiferes Yisrael Avos 6:50; Ben Ish Chaiy Ki Seitzei 10; Yabia Omer 1 Y.D. 18; Pesakim Uteshuvos 242:41 footnote 312; 243:8 footnote 56