[Buggman]

It's a complicated issue, and I don't want to dig into the precise chronology just a the moment. The general issues that lead us to believe in a gap are as follows:

1) There is no mention of a making or confirming a covenant specifically for 7 years (the 70th Week) in the NT.

2) There is no mention that sacrifice and offering were brought to an end at the time of the Crucifixion; they went on for 40 more years--in fact, it can be demonstrated that Sha'ul himself paid for the sacrifices for four Messianic Jews and possibly for himself as well in Ac. 21.

3) As a corallary, in both of the other occassions that Daniel speaks of the sacrifices being brought to an end (in chapters 8 and 11), he describes it as an evil thing done by the enemy of God (Antiochus and/or the Antichrist), so it doesn't make sense to assume that the ending of sacrifice in Dan. 9:27 is a good thing done by Yeshua.

4) The time between the order to rebuild Jerusalem (Neh. 1) and the coming of the Messiah King was not 490 years, but 483, which leaves us a week.

5) The six promises made to Israel (Daniel's people) and Jerusalem (Daniel's holy city) in v. 24 were not completed. The interpretation that turns them into a threat ("You have 490 years to shape up, or else!") is untennable for other reasons, not the least of which is that it would turn all of God's promises to Israel in the Tanakh into a lie.

6) The first 69 weeks have a starting point (the order to rebuild Jerusalem) and an ending point (the comming of the Messiah King). The 70th Week likewise has an independant starting point (the 7 year covenant) and an ending point (the fulfillment of the promises to Israel and Jerusalem), which suggests a gap.

7) Likewise, the prophecy puts two events--the Crucifixion and the destruction of Jerusalem and the Temple--in between the 69th Week and the 70th. This again implies a gap of at 40 years at the very least.

8) Many of the ante-Nicean Church fathers recognized the gap:

This “gap theory” of the Seventieth Week is by no means a new interpretation, as some have contended. Irenaeus alludes to a future Seventieth Week when he says, “Now three years and six months constitute the half-week.”[1] A sometimes contested quote from Hippolytus expands on this as follows:

For he says, “I shall make a covenant of one week, and in the midst of the week my sacrifice and libation will be removed.” For by one week he indicates the showing forth of the seven years which shall be in the last times. And the half of the week the two prophets, along with John, will take for the purpose of proclaiming to all the world the advent of Antichrist, that is to say, for a “thousand two hundred and sixty days clothed in sackcloth.”[2]

Victoranus also indicated that the Seventieth Week was yet future when he wrote,

“They shall tread the holy city down for forty and two months; and I will give to my two witnesses, and they shall predict a thousand two hundred and threescore days clothed in sackcloth.” That is, three years and six months: these make forty-two months. Therefore their preaching is three years and six months, and the kingdom of Antichrist as much again.[3]

Admittedly, other church fathers like Tertullian, a contemporary of Hippolytus, believed that all Seventy Weeks had been fulfilled, so the above quotes should not be taken as universal agreement among the Ante-Nicean church fathers on this matter. However, they do show that the expectation of a future Seventieth Week did not begin only in the 1800s with the rise of Dispensationalism, as many amillennialists charge. And since Hippolytus was Irenaeus’ student, Irenaeus Polycarp’s, and Polycarp an original student of the same Yochanan the Emissary who penned the Apocalypse, it would not be too far of a stretch to say that the idea of a future Seventieth Week was almost certainly taught by the Emissary to his students as well.

References:
[1] Irenaeus, Against Heresies, Book V, chapter 25.3
[2] Appendix to the Works of Hippolytus, 21.
[3] Victoranus, ibid. chapter 11.3

Now, there are scholars who disagree and we can argue about some of the details of chronology, like the exact year and so forth, but that's the basis for believing in a gap between the two periods.

[P-Marlowe]

I think one of the principle reasons why the Ante-Nicean fathers may have considered that the Book of Revelation was history rather than prophecy is simply because Jerusalem had been destroyed and the Jews were dispersed all over the planet. Hence there simply was no nation of Israel and no prospects for it to ever be a Nation again. Hence the rise of replacement theology and Preterism.

But to consider it history is to make the book so allegorical as to be unintelligible. The book is written as if the events actually happen as they are described. Numbers of days, numbers of years, numbers of casualties, amounts of world wide destruction, plagues, famines, weather disasters, comets, giant locusts and freaks of nature.... yet no reading of history (even in the most allegorical sense) will coincide with the predictions in the Book of Revelation. If, in fact, the book of Revelation is supposedly complete, then it would appear to me that whoever wrote it just got it wrong. IMO, it would not simply not pass the test of a prophet in Deuteronomy.

IMO either the book of Revelation is for the most part yet future, or it is not an inspired work. Since I believe every word of it to be the inspired words of God, the historical option is simply not tenable.