At the end of this week's Parsha, there is a very strange yet interesting happening. The Posuk tells us that once the Mishkan was completed and the dedication ceremony was finished, Hashem's "ענן," representing His Shechinah, covered the Mishkan and "כבוד ה'," the Honor or Glory of Hashem, filled the Mishkan (שמות מ':ל"ד). Immediately after this, the Posuk tells us that Moshe was unable to go into the Mishkan, because the Shechinah was resting on it, and the Honor or Glory of Hashem had thus filled the Mishkan (שם פסוק ל"ה). Many Meforshim are bothered by this Posuk and wonder how it could be true. After all, this was the same Moshe concerning whom the Posuk tells us "ויבא משה בתוך הענן," meaning that Moshe came in to where the Shechinah was (שמות כ"ד:י"ח) when he was on Har Sinai. Why was Moshe able to go to into the Shechinah on Har Sinai, yet he was not able to enter into the Shechinah in the Mishkan?
The Alshech comments that the Shechinah residing on the Mishkan was a much stronger form of the Divine Presence than was the Shechinah on Har Sinai, for the Shechinah in the Mishkan had assumed an additional dimension, which was so awesome that even Moshe could not enter. This extra dimension is apparent, says Rabbeinu Avraham Ben HaRambam, citing Rabbeinu Saadyah Gaon, by the Torah's use of the term ענן ה'"," "the Cloud of Hashem," regarding the Mishkan, as opposed to just "ענן," "the cloud," which was used regarding Har Sinai.
This extra dimension, according to the Klei Yakar and the Ohr HaChaim (שם מ':ל"ד), was reflected by the combination of the usual "ענן," (the Shechinah), and this extra dimension of the "כבוד ה'." Previously, on Har Sinai, there was only the ענן component, and therefore Moshe was able to enter into it. But now, with the addition of the כבוד ה', even Moshe was forbidden to enter (until Hashem gave him explicit permission). There is a similar notion to this found in Sefer Melachim Aleph (ח':י"א), when Shlomo HaMelech builds the Beis HaMikdash. The Posuk tells us that both the ענן component and the כבוד ה' component entered into the Beis HaMikdash, and all the people had to be evacuated from the premises, because no one could be in the Beis HaMikdash at this time, just like Moshe was unable to be in the Mishkan.
Along the same lines, the Netziv, is his Peirush on Chumash called HaAmek Davar (שם מ':ל"ה), says that this resting of the Shechinah on the Mishkan was different from any of the Shechinah's past dwellings. For previously, the Shechinah had always dwelt "על," on, as seen for example, regarding the events of Har Sinai, where the Torah says "וענן כבד על ההר" "a heavy cloud (the Shechinah) was on the mountain." As for the Mishkan, though, the Shechinah dwelt not only on the Mishkan, but in it as well. Thus, this extra dimension of the Shechinah caused Moshe to be unable to enter.
Rav Shimon Schwab, in his Sefer Maayan Beis HaShoeivah (לשמות שם פירוש ב'), writes that there was another big difference between the Shechinah's dwelling on Har Sinai and on the Mishkan. The Gemara in Kesubos (דף ה.) comments that "גדולים מעשה צדיקים יותר ממעשה שמים," "the actions of a righteous person are greater than the acts of the Heavens." The Gemara cites proof for this from the Posuk which says "יסדה ארץ אף ידי," "My hand has laid the foundation of the earth" (ישעיה מ"ח:י"ג), referring to the שמים מעשה, while regarding the מעשה צדיקים, the actions of righteous people, the Torah says "מקדש ה' כוננו ידיך," "the Sanctuary of Hashem your hands have founded" (ט"ו:י"ז שמות). From the fact that the Pesukim assign only one "hand" to the מעשה שמים, and give to the מעשה צדיקים more than one "hand," the Gemara learns the works of righteous people are greater than those of Hashem. Therefore, says Rav Schwab, the Shechinah which was established by Hashem on Har Sinai was not as strong as the Shechinah established by the actions of the righteous, for after all, it was Bnai Yisrael who built the Mishkan and caused the Shechinah to descend into it.
Rav Moshe Feinstein, in his Sefer Darash Moshe (שם מ':ל"ה), perceives the Shechinah as being the same Shechinah that rested on Har Sinai; but, he warns, one should not see this as lowering of Moshe's status just because previously he was able to go where the Shechinah was and now he could not. Rather, this was simply a sign that Moshe should stay away from the Mishkan while Hashem "transferred" His Divine Presence into the Mishkan, a process to holy for even Moshe to bear witness to.
The Ramban (שם) dismisses this question almost entirely. He does not recognize this "extra dimension" of the Shechinah; rather, he writes, in both situations Moshe at first was unable to enter due to the presence of the Shechinah. After all, concerning Har Sinai, the Posuk states "והר סיני עשן כולו," indicating that the entire mountain was filled with the Shechinah (י"ט:י"ח שם), just like we find by the Mishkan, where the כבוד ה' filled the entire dwelling. And any time the Shechinah fills an entire area, Moshe is not allowed to enter. It is only when Hashem calls him and gives him express permission to enter, as stated by the Posuk in Parshas Mishpatim (שם כ"ד:ט"ז), that Moshe dares to enter (פסוק י"ח שם). Similarly, says the Ramban, only in the beginning of Parshas VaYikra, where we read "ויקרא אל משה," "Hashem called to Moshe," and we see that He gives him permission to enter, only then was Moshe able to enter the Mishkan.
It is interesting to note that Rav Schwab (שם) comments that the words which describe that Moshe was "unable" to enter, "ולא יכל," imply that this restriction was placed by Moshe himself on himself. For Moshe, after completing the building of Hashem's "residence," was now acting in his usual way, one of humility, as the Posuk tells us later "והאיש משה עניו מכל האדם אשר על פני האדמה," "and Moshe was more humble than any man ever on earth" (במדבר י"ב:ג'). Moshe was thus letting the Shechinah get all the Kavod and attention, while he himself, the builder and architect, waited outside, far away, in humility.
Finally, there are some who resolve this question based on the Gemara in Yoma (דף ד:), which points out that the Posuk uses the word "בתוך," "in the midst of" to explain Moshe's ascension into the midst of, the Shechinah on Har Sinai, and then compares the use of this word here to its use in Parshas Beshalach, regarding the crossing of the Red Sea. The Posuk there tells us that Bnai Yisrael walked בתוך, in the midst of, the sea (שמות י"ב:כ"ב), which does not mean that they walked through the sea, but rather that the sea became a wall for them and made room for them to walk through. So too, when Moshe came בתוך, in the midst of, the Shechinah on Har Sinai, he did not enter into the actual Shechinah itself (for no human can, not even Moshe), but rather, there was a path carved out for him within the Shechinah, and he was בתוך the Shechinah. While he was geographically in the midst of the Shechinah, therefore, he was not actually in it in reality. Moshe thus had the ability to enter into the Shechinah only if there was room made for him to stand; at the completion of Mishkan, though, there was no room for him, for the כבוד ה' filled the entire area, and he was unable to enter the Mishkan. But on Har Sinai, where there was a place set aside for him, he could enter.