Maybe no individual ECF held that ALL Eschatology was fulfilled in 70AD However, when we accumulate all the individual prophesies that any given ECF on their own DID believe to be fulfilled in 70AD, and put them together, we arrive very near a consistent preterist position, even if they were personally inconsistent on their application thereof.
For certain, the greatest number of the earliest Christians believed that a number of, if not all, prophecies of the Olivet Discourse were fulfilled in the first century destruction of Jerusalem.
The challenge, in fact, is to find even one early Christian that didn't teach the Preterist interpretation of Matthew 24. The earliest and most significant writers were in
unanimous agreement, proclaiming the fulfillment of these prophecies in the time of the AD70 destruction of the Jewish city, temple and nation.
Here's a snippet:
Origen - Against Celsus | John | Matthew
"I challenge anyone to prove my statement untrue if I say that the entire Jewish nation was destroyed less than one whole generation later on account of these sufferings which they inflicted on Jesus. For it was, I believe, forty-two years from the time when they crucified Jesus to the destruction of Jerusalem."
Chrysostom - Homilies on
Matthew 24 "Was their house left desolate? Did all the vengeance come upon that generation? It is quite plain that it was so, and no man gainsays it."
Chrysostom - St. Chrysostom's Liturgy
"Having in remembrance, therefore, this saving commandment and all those things which have come to pass for us: the Cross, the Grave, the Resurrection on the third day, the Ascension into heaven, the Sitting at the right hand, and the second and glorious Coming"
The ECFs recognized:
(1) that the great tribulation is past, transpiring at AD 66-70
(2) that AD 70 involved a coming of Jesus Christ in judgment
So, while they did not establish a biblically consistent preterism, they were far more preteristic in their understanding of eschatology than most modern futurists. The fact is that the ECFs had their hands full with formulating a consistent Christology (the nature of Christ and the Trinity), and didn't spend as much time formulating an orthodox, systematic eschatology. We know that the ECFs had mostly assigned
Matthew 24 to the past, and the Protestant Reformers had a majority view that all
Matthew 24 was fulfilled in the first century.
Classical preterism (i.e. The Catholic Preterism of the likes of James Aiken, Scott Hahn, St Cryssostom, St Thomas Aquinas, Eusebius, etc...) sees AD 70 as a temporal judgment-coming of God/Christ.
St. Thomas Aquinas, St. Augustine, St. Eusebius all understood this basic principle of bible eschatology, and we really ought to take their words to heart.
As St. Thomas Aquinas taught:
The signs of which we read in the gospels, as Augustine says, writing to Hesychius about the end of the world, refer not only to Christ's [future] coming to judgment, but also to the time of the sack of Jerusalem, and to the coming of Christ in ceaselessly visiting His Church. So that, perhaps, if we consider them carefully, we shall find that none of them refers to the coming advent, as he remarks: because these signs that are mentioned in the gospels, such as wars, fears, and so forth, have been from the beginning of the human race (Thomas Aquinas; Summa Theologica, Supplement Question 73, Article 1)
And even St. Gregory, Bishop of Nyssa (AD 336-395)
"Do we romance about three Resurrections? Do we promise the gluttony of the Millennium? Do we declare that the Jewish animal-sacrifices shall be restored? Do we lower men's hopes again to the Jerusalem below, imagining its rebuilding with stones of a more brilliant material? What charge like these can be brought against us, that our company should be reckoned a thing to be avoided?"
Regardless, it's the views of the ECF from AD30-70 who's views we ought to prefer when they are shown to contradict the views of the Later ECF.