The way this is derived is from a penetrating analysis of his Chumash Nesivos Shalom's commentary, which was composed by 5 people, and anthologized from Rashi, Rashbam, Ibn Ezra, Ramban, with original exegesis by these men as well. Everyone knows that the classical exegetes interpreted verses in varied ways, sometimes agreeing and supporting one another, sometimes disagreeing. There are a number of places in the Torah where one exegete will interpret a verse in a Messianic sense, but another doesn't. So, says this analysis, when Rashi interprets one verse as referring to the Messiah, it will adopt the Rashbam who didn't. When Rashbam interprets a verse messianically, it will adopt Rashi, and in this way the Moshiach is missing from the Bi'ur.
Here is how R. Yitzhak Nissenbaum put it (Alei Cheldi p. 96):
המשכילים הללו פרשו את התורה לפי הפרושים המסורתיים, אלא מה עשו? הטו לצדדין, למשל, כך: בענין האמונה בביאת המשיח, מבאר רש"י את הכתוב, ,,עד כי יבא שילה" על משיח, והרשב"ם מבאר אותו על זמן אחר, לעומת זה באר הרשב"ם את הכתוב ,,דרך כוכב מיעקב" על מלך המשיח ורש"י מבאר אותו על דוד. בין כה וכה גם ברש"י וגם ברשב"ם יש זכר בתורה לביאת המשיח. ומה עשו בעלי ה,,באור" הם בארו את הפסוק הראשון כרשב"ם ואת הפסוק השני כרש"י, ובין כה וכה אין ב,,באור" כל זכר לביאת המשיח
These Maskilim explained the Torah according to traditional commentaries, but what did they do? They marginalized traditional viewpoints, for example, with regards to the belief in the coming of the Messiah, Rashi interprets Gen. 49:10 to refer to the Messiah, but Rashbam interprets it as referring to another time period. On Numbers 24:17 Rashbam interprets it to refer to the Messiah, but Rashi explains it as referring to King David. Either way, there is a reference to the coming of the Messiah in the commentary of both Rashi and Rashbam. The commentators of the Bi'ur explained the first verse according to the Rashbam, and the second according to Rashi. Either way, there is no reference to the Messiah in the Bi'ur.Similarly, I am told that this is reported in the name of the 'Minsker Gadol,' R. Yeruhem Leib Perlman. In that case, the second verse cited is Deut. 33:5, which Ramban interprets messianically, and Rashi explains it otherwise. Since it is unlikely that Rabbi Nissenbaum was the one to discover this, it's safe to assume that this was something which was noted by many, whomever first discovered it.
The first question is, is this entirely accurate? Is there no reference to the Messiah in the Bi'ur? Although I have not discovered such a reference, I believe it is notable to point out that Mendelssohn specifically writes (in a paranthetical comment to Wessely in Lev. 26:39) that the Jews were exiled from their land by God for their sins. The continuing travails of the exile are natural, since sons cannot be punished for the sins of the fathers. However, only a miracle by God can restore the Jews to their land, even if the present conditions of the exile are natural, rather than supernatural. As far as I can tell this is awfully close to a reference to the coming of the Messiah, and it is literally written by Mendelssohn himself.
The red brackets is his paranthetical insertion, and the arrows are the comments I wish to highlight.
Putting this aside - although I believe the post could really end there - here are some objections to the original suggestion.
First, five people wrote the commentary, so it is hard to see how there could be a unified policy. The first example is from Genesis, and it was written by Solomon Dubno, whom no one would accuse of not believing in the Messiah. Although I do not know enough about the author of the Numbers commentary, Aaron Freudenthal of Jaroslaw, it's is difficult to suggest that he interpreted there according to Rashi in order to ensure that there would be no reference to the Messiah in the book. Thus, we have to dismiss the suggestion that any one individual commentator interpreted his verse non-messianically because he personally did not believe in it, because this idea requires more than one verse. The only other possibility is that Mendelssohn himself is responsible for this through editorial interference. While one could posit this, one could posit anything. We should like some evidence. As we see from the Wessely example, he carefully noted where his own hand touches the commentary.
Second is how loaded was the suggestion that there is no future coming of the Messiah. In his time, this could only be understood by the many, many gentile scholars looking over Mendelssohn's shoulder as meaning that the Messiah already came. This is something which surely he wished to avoid at all costs, and it is hard to say that this theme was therefore presented on purpose. Indeed, this is likely the reason why Dubno, for example, would interpret ad ki yavo shilo according to Rashbam, a verse which is given a christological interpretation. Thus it is not so understandable if the book avoids messianic interpretation, even intentionally.
Aside for this, we should examine Mendelssohn's other writings to see if or how he refers to the Jewish belief in the coming of the Messiah. An interesting case in point is his book Jerusalem. I will use the 1838 English translation by M. Samuels. In volume 2 page 44, he writes:
In the system of man's duties, the duties to God, in the main, form no distinct division. All the duties of man are duties to God, some concern ourselves, some our neighbours. Out of love of God we are rationally to love ourselves, his creatures ; out of rational love of ourselves we are bound to love our neighbours.In other words, it will be good for people if finally, in the year 2240, they will come around and stop acting against the obviously correct principles that God really wants people to be good to one another.
[God] only designs our good, the good of every individual . . .
This common-place is so trite, that good sense is surprised that people could ever have been of a different opinion. And yet mankind have, from the beginning, acted against those plain principles! Well will it be for them, if in the year 2240 they leave off doing so.
What is the year 2240? Alexander Altmann ingeniously noted that it is the year 6000 according to the Jewish calendar, the year posited by the rabbis as the final date for the Messiah's arrival. Here is what he writes in his notes to Alan Arkush's translation of Jerusalem:
Although I would agree that we cannot deduce his belief or lack of belief in a literal coming of the Messiah from this comment, it surely is interesting.
However, we should see that it is not so simple that it refers to the years 6000 (cleverly called 2240). Although Altmann was not the first person to do the math - it was already written this way in the Hebrew edtition translation by Abraham Dov Baer Gottlober (Zhitomir 1866):
apparently Altmann overlooked the note in the 1838 edition, for he did not even mention it to dismiss it. Here is what it says:
Note 8: L'an Deux Mil Deux Cent Quarante, by Mercier.Thanks to the miracles of modern research this can be explained in 10 seconds. It refers to Mercier's 1771 book The Year 2440: A Dream If Ever There Was One (which, funnily enough, was changed to 2500 in the English edition). Wikipedia helpfully describes it as "a utopian novel set in the year 2440. . . . [it] describes the adventures of an unnamed man, who, after engaging in a heated discussion with a philosopher friend about the injustices of Paris, falls asleep and finds himself in a Paris of the future."
Thus, it is pretty clear that in 1783 when Jerusalem was released "the year 2440" was a well-known reference to the literati of Europe. It meant "the future."
Edited 12.1.11: I was never good with numbers, and a sharp-eyed commenter pointed out that the book uses the year 2440, while Mendelssohn wrote 2240. Therefore Altmann's point (preceded by Gottlober) is stronger. While I still feel that Mendelssohn was almost certainly alluding to Mercier's book, he also changed 2440 to 2240, which is indeed the year 6000, and therefore he is alluding to the yemot ya-mashiach.
Be that as it may, I ask you, is this the face of a man who didn't believe in the Messiah?