¶ 1 The Navi said in Chagai 2:9 that גדול יהיה כבוד הבית הזה האחרון מן הראשון, the glory of this Temple, the Acharon, will be greater than that of the first. "Although the First Temple has been destroyed, another will be built, and it will be greater than the first." The Temple he is prophesying is the Second Temple, as the Gemara in Bava Basra 3a says: The "greater glory" of the Second Temple was only temporal, in that it was taller and lasted longer than the first.
¶ 2 Many, many people have asked that the term אחרון seems terminal; we generally assume that אחרון means "the last one," and since that Temple was ultimately destroyed, Chagai's prophecy would seem to dash our hope for a third and final Beis Hamikdash.
This question first appears in the Zohar פי פנחס דף רכא.:
כלום, ותירץ לתלמידיו אחרון בידי אדם אבל מקדש שלישי יהיה בידי שמים כמו שנאמר [זכריה בי, ט/] ואני אהיה לה נאם ה׳ חומת אש סביב
It's worth knowing that The Beis Halevi in Toldos says a teretz (here paraphrased) along the same lines as the Zohar:
בית המקדש הניתן לישראל שיבב״א, והבתים הראשונים הם נקראים מוקדמים, שהם רק הכנה לעיקר הבית שהוא השלישי. נמצא דהבית השני הוא אחרון למוקדמים, וזה שאמר הכתוב גדל יהיה הבית הזה חאחרון למוקדמים
So according to the Zohar's teretz and the Beis Halevi's teretz, אחרון does mean the last one, but it was the last of its kind. The first two Temples were made by the human hand, and the third will be of divine fire and categorically different.
After the Zohar, the next person that addressed the question is Rashba, in his Teshuvos שו״ת ח״ד סימן קפ״ז:
הנה הכתוב אומר שאין בית אחר הבית השני, שכן כתוב: ״גדול יהיה כבוד הבית הזה האחרון מן הראשון״. הנה שקראו אחרון׳ ואם יש אחר לאחריו׳ לא יקרא לזה אחרון. אמרתי לא קראו אחרון אלא בהצטרף אל הראשון, וכמוהו ״וישם
את השפחות ואת ילדיהן ראשונה, ואת לאה וילדיה אחרונים, ואת רחל ואת יוסף אחרונים/
הנה קראה ללאה וילדיה אחרונים אע״פ שרחל ויוסף אחרונים להם, אלא שקראם אחרונים
״בהצטרף אל השפחות וילדיהן״. אמר: זה בדיבור בני אדם שאינם יודעים ולא בידם האיחור,
אבל כדברי הנביא שיודע האמת לא יבא. אמרתי :בא אפילו במה שיאמר לנביא מפי הגבורה׳ שכן
כתוב במה שאמר הוא יתעלה למשה ,׳והיה אם לא יאמינו ולא ישמעו לקול האות הראשון והאמינו
לקול האות האחרון, והיה אם לא יאמינו גם לשני אותות האלה ולקחת גם ממימי היאור״, זעה
שקרא לאות השני אחרון אע״פ שבא אחריו אות המים׳ עכ״ל
The Gemara in Nazir 21a addresses the question as well, because the Mishna on the previous daf says the following: If "Person A" verbally promises to become a nazir, and Person B says "And I," and Person C says "And I," they are all viewed as having enunciated the formula that makes a person a nazir. However, the status of the latter people depends on that of the prior. So, the Mishna says, if A annulled his status of Nazir, the verbal promise he made is rendered retroactively meaningless, so the people who followed and said "And I" are also released from being Nazirs. If the Acharon person annuls his promise, he alone is released, but the others are unaffected. Amoraim argue in the Gemara about whether the last person is dependent on the first (C means "I am like A") or on the one that immediately preceded him (C means "I am like B.") The machlokes would be relevant where there are A B and C, and B annulled his Nezirus. If C is dependent on B, C is also annulled. If, however, C is dependent on A's declaration, then C remains a Nazir, because A's declaration has not been voided. The Gemara attempts to prove that C is dependent on B by showing that the Mishna only said that the annulment of the Acharon affects no one else, which implies that an annulment by B would affect others, namely, C. The Gemara rejects the proof; it could be that when the Mishna says that annulment by the Acharon affects no one else, it means annulment by B. Why, the Gemara asks, would we call B the Acharon? Because, the Gemara answers, when you say Rishon, the one that follows is called Acharon, even if there are others that follow. In other words, Acharon means "later," or "the next", and it does not have to mean "the last," but only where the word Rishon was used. (This is pointed out by the Rashash there in Nazir.)
This can also be seen in דברים כט כא where it says הדור חאחרון בניבם אשר יקומו מאחריכם
There, too, Acharon means "those that follow."
I think that the the best explanation of the passuk in Chagai (and the pesukim in Shemos where Hashem told Moshe about the Os Ha'acharon) is that of the Malbim in Chagai:
זה הוא תנאי, אם יבא עוד רוח אחת, והוא אם יעשו את הדבר אשר כרת עמם בצאתם ממצרים,
אז יהיה הוא הבית האחרון שאין אחריו בית, ויהיה גדול מן הראשון, וכשלא נתקיים התנאי כי
לא שמרו ברית לא היה שלום במקום, כי נתרבו הפריצים והכתות ושנאת חנם, לא נתקיים
היעוד אז, עד שיבנה הבית האחרון בקץ האחרון שאז יתקיימו היעודים האלה,
The point of the Malbim is that words in Tanach, and the words of Nevu'ah, intentionally encompass a multiplicity of meanings. When Chaggai described the Bayis Sheni as the Acharon, the word held several possible meanings, ranging from benign to malign. One of those several meanings would definitely come true (like, among many other examples, the story of Rabbah bar Rav Huna chair in Gittin 35). Which of the meanings will come true- that depends on our behavior. Chaggai was telling Klal Yisrael that if they will be worthy, the Second Temple would be the Acharon/Ultimate. If they will be unworthy, then it would be the Acharon/Next.
(It has been said that the Malbim's approach depreciates (see note) the entire idea of nevuah, because it makes nevuah meaningless- saying that many possible outcomes can be read into the nevuah makes it tentative and ambiguous, which means it's not a prediction at all- "I predict that something will happen tomorrow." I don't think that's a criticism, because the Navi is saying something absolute- that a Bayis will soon be built. The vagueness is only regarding the future of the Bayis. It's not like the Yarum Roshcha of the dreams Yosef interpreted. As pointed out by g in the comments, this is a universally accepted concept, first seen when we were forewarned of four hundred years of exile and servitude at the Bris Bein Habesarim, but the counting commenced long before they went to Mitzrayim, so the literal avdus only lasted two hundred and ten years. Also, by Yonah and Ninveh, נהפכת was a threat that meant "turned upside down," i.e., physical upheaval. But after they did teshuva, it turned out to mean spiritual upheaval, i.e., rebirth. But please see this post on Parshas Bo where I bring the Beis Halevi- and, oddly, the Malbim- that discuss this point in depth, and which argues that while Nevua may be protean as to the ultimate fact, it is specific as to intent, )
¶ 3 The reason all this is here, in this week's parsha, is because of a passuk in this parsha - Shemos 12:13, וְהָיָה הַדָּם לָכֶם לְאֹת עַל הַבָּתִּים Many achronim have used this passuk to explain the passuk in Chagai.
¶ 4 Two weeks ago, In parshas Shemos, we read about the miracles Hashem gave Moshe to show Bnei Yisrael. First, (4:3) there was the miracle of the staff that turned into a snake. Then Hashem gave Moshe another miracle for a sign, which was Moshe's hand becoming leprous and being healed (4:6). Hashem said (4:8) that if the Bnei Yisrael don't believe the first miracle, the snake, then they will believe the "Os Ha'acharon," the sign of the leprosy. Afterwards (4:9), Hashem said that if Bnei Yisrael don't believe either of the miracles, there was a third one, turning a cup of water into blood. So it is evident that when Hashem called the second of the three miracles the "Acharon," it only meant "the next one," not the final one. In this week's parsha, Hashem told the Bnei Yisrael to put blood from the Korban Pesach on their doorposts; the passuk says that the blood will be a sign on the houses. Literally, this means that the blood of the korban will designate and make safe the Jewish dwellings. But in the context of what we've been discussing, it can be understood to mean that the blood, the third of the three signs Hashem gave Moshe, is proof that there will be a third Beis Hamikdash, because there, the word Acharaon only meant "next" and not "final," so in Chagai also, although the second Bayis is called "Acharon," it only means "the second one", because there will be a third. The blood will be a sign regarding the houses, i.e., the Temples.
I found that the Malbim's idea is found in Rashi in Yechezkel 43:10-11 and 43:14-15.
Surgeon General's Warning:
This is a vort. It is not meant as a pshat, or a remez, or a drush, or a sod. It's just a vort, what we call שעשוע בדברי תורה, in the sense of לולא תורתך שעשועי . It has been said by many people. It's impossible to know who said it first, but none of them meant it to be interpretive.