The actual prophetic answer (you asked!): So it can be desecrated by the anti-christ.

https://www.biblegateway.com/passage/?search=Matthew+24&version=NKJV

You're looking for verse 15 in the link above, which is less clear than most Christians will admit. But, if you're actually interested, here's what leads up to this and why Christians assume a third temple will be built.

It was prophesied that the Jews would lose their land and one day return to Israel. Amos 9:14–15

I will bring back the captives of My people Israel; They shall build the waste cities and inhabit them; They shall plant vineyards and drink wine from them; They shall also make gardens and eat fruit from them. I will plant them in their land, And no longer shall they be pulled up From the land I have given them,” Says the Lord your God.

It's actually talked about a lot (also see Isaiah 41:9, Isaiah 43:5–6, Ezekiel chapters 37-39), and the whole thing is kind've a wild prediction given the timeline.

Bear in mind that the Christians believe the Jews are currently spiritually blind and separate from God. They still support the Jews because the bible says that those who bless Israel will be blessed and to pray for peace in Jerusalem.

So, the Jews are going to build the temple because well, they're divinely driven to do so...

“And he [the prince] will make [some translations say ‘confirm’] a firm covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering; and on the wing of abominations will come one who makes desolate [some interpretations say, set up an idol on the wing or precipice of the Temple], even until a complete destruction, one that is decreed, is poured out on the one who makes desolate.” (Daniel 9:27; see also 2 Thessalonians 2:4)

...Christians believe that, though Jerusalem and Damascus will be destroyed, this whole process will eventually lead to the Jews (the ones that survive the End times... The bible seems to suggest most won't) to accept Jesus as the true messiah.

Anyway, it's a lot, and there is loads more. But for me, if you've read biblical prophecy to any significant extent... It gets real hard to think we're not living near the end times it describes, and a lot of it happens to involve Israel.

The actual prophetic answer: So it can be desecrated by the anti-christ.

https://www.biblegateway.com/passage/?search=Matthew+24&version=NKJV

You're looking for verse 15 in the link above, which is less clear than most Christians will admit. But, if you're actually interested, here's what leads up to this and why Christians assume a third temple will be built.

It was prophesied that the Jews would lose their land and one day return to Israel. Amos 9:14–15

I will bring back the captives of My people Israel; They shall build the waste cities and inhabit them; They shall plant vineyards and drink wine from them; They shall also make gardens and eat fruit from them. I will plant them in their land, And no longer shall they be pulled up From the land I have given them,” Says the Lord your God.

It's actually talked about a lot (also see Isaiah 41:9, Isaiah 43:5–6, Ezekiel chapters 37-39), and the whole thing is kind've a wild prediction given the timeline.

Bear in mind that the Christians believe the Jews are currently spiritually blind and separate from God. They still support the Jews because the bible says that those who bless Israel will be blessed and to pray for peace in Jerusalem.

So, the Jews are going to build the temple because well, they're divinely driven to do so...

“And he [the prince] will make [some translations say ‘confirm’] a firm covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering; and on the wing of abominations will come one who makes desolate [some interpretations say, set up an idol on the wing or precipice of the Temple], even until a complete destruction, one that is decreed, is poured out on the one who makes desolate.” (Daniel 9:27; see also 2 Thessalonians 2:4)

...Christians believe that, though Jerusalem and Damascus will be destroyed, this whole process will eventually lead to the Jews (the ones that survive the End times... The bible seems to suggest most won't) to accept Jesus as the true messiah.

Anyway, it's a lot, and there is loads more. But for me, if you've read biblical prophecy to any significant extent... It gets real hard to think we're not living near the end times it describes, and a lot of it happens to involve Israel.