Dating the Gospels

I. Conventional reasons to date the Gospels after 70 AD:

1. Form criticism

According to this theory, the sources of the gospels underwent extensive creative oral development before commitment to writing. Other issues aside, many NT scholars date the Pauline letters much earlier than the Gospels, yet if Christians could write letters in the 40s-60s, there's nothing to inhibit them from writing Gospels in the 40s-60s. So the form critical stipulation is arbitrary. 

2. Olivet Discourse

i) Liberal scholars don't think Jesus could foresee the fall of Jerusalem. Therefore, the Synoptic Gospels had to be written post-70 AD. Given Markan priority, that pushes Matthew and Luke further out. If, however, you accept the supernatural phenomenon of precognition, not to mention the deity of Christ, then that objection reflects unjustified naturalistic prejudice. 

ii) A more specific objection is that Luke's version of the Olivet Discourse (Lk 21:20-24) reflects knowledge after the fact. Luke allegedly rewrote the oracle with the benefit of hindsight. By way of response:

iii) Even on naturalistic grounds, the account uses stock siege warfare imagery from the LXX. 

iv) Jerusalem was a fortified city, so siege warfare would be the standard tactic.

v) This wasn't the first time Jerusalem had been surrounded by foreign armies (e.g. Babylonians, Assyrians, Romans). 

vi) Luke has his own sources, independent of Matthew and Mark. All of them may well be quoting what Jesus said, but excerpting different statements. Cf. D. Wenham, The Rediscovery of Jesus' Eschatological Discourse (Wipf & Stock 2003). What Luke records is more germane to his Gentile target audience while what Matthew records is more germane to his Jewish target audience. 

3. John's Gospel is more theological advanced than the Synoptics 

In a sense that may be true. However, this doesn't imply that his Gospel is later than the Synoptics–although it may be. For example, you can have two contemporaries who write about a war they lived through. One account may be more insightful than another. That has nothing to do with relative chronology. Moreover, John's Gospel uses Jewish categories and OT paradigms to express theology. 

II. Reasons to date the Gospels before 70 AD:

1. Authorship

i) If traditional authorship is correct, then that sets an outer limit for the composition of the Gospels inasmuch as they had to be written within the lifetime of Matthew, Mark, Luke, and John. Minimally, that rules out the 2C. 

So that depends on evidence for traditional authorship, which is varied, including both internal and external evidence. One argument is the titles of the Gospels. Our Greek manuscripts are remarkably consistent in their authorial ascriptions. But it's hard to account for that uniformity if the titles are late editorial additions, considering the fact that ancient Christian scribes worked independently of each other. So that implies the originality of the titles. Detailed arguments are provided by scholars like Hengel and Bauckham. Cf. R. Bauckham, Jesus and the Eyewitnesses (Eerdmans, 2nd. ed., 2017); M. Hengel, The Four Gospels and the One Gospel of Jesus Christ (Trinity Press 2000). (Not that they affirm traditional authorship, which is ironic, and reflects a failure to follow through with the logic of their own arguments.)

ii) There's a sense in which authorship is more important than date. So long as the Gospels reflect living memory. So long as they were written by people who knew Jesus or knew people who knew Jesus. 

2. Historical accuracy

If the Gospels were written by people who were not eyewitnesses or didn't have access to eyewitness sources, then it's very hard to explain the historical accuracy of the Gospels. Cf. Peter Williams, Can We Trust the Gospels? (Crossway 2010). So that implies authors with firsthand knowledge. 

3. The date of Acts

The Book of Acts ends abruptly, without informing the reader about the fate of Paul. There's a steady buildup to Paul's impending trial before Caesar, only to leave that hanging in midair. The most natural explanation for lack of resolution is that Acts was written before the final disposition of Paul's case. For a classic exposition and defense, cf. C. Hemer, The Book of Acts in the Setting of Hellenistic History (Eisenbrauns 1990), chap. 9. Readers are bound to be curious about Paul's fate. Although that argument is less popular among scholars than it used to be, it's still the most plausible, straightforward explanation. In addition, Acts lacks any reference to the demise of Peter and James (brother of Jesus), even though it records the demise of other church leaders (Stephen, James bar Zebedee). Assuming that Acts was written before Paul's execution, that pushes Luke's Gospel further back. 

4. The Synoptic Problem

i) On a conventional solution to the Synoptic Problem, Matthew and Luke made use of Mark. That entails Markan priority. The basic argument is that if a teacher read three student papers as similar to each other as the Synoptics, he'd logically conclude that there was collaboration or literary dependence. This doesn't mean Matthew and Luke are necessarily dependent on Mark for on their information, even in parallel accounts. There is evidence that they had their own sources of information, even in parallel accounts. Cf. L. McGrew, Hidden in Plain View: Undesigned Coincidences in the Gospels and Acts (DeWard 2017). 

ii) Even if that yields a relative chronology, it doesn't give an absolute chronology. But it provides a rough terminus ad quo and terminus ad quem. At one end, Mark could be as earlier as the 40s. Cf. https://legacy.tyndalehouse.com/tynbul/Library/TynBull_1972_23_04_Wenham_PeterInRome.pdf

At another end, if Acts was written before Paul's execution, then Luke was probably written around the late 50s, give or take. I have no opinion as to whether Matthew was written before or after Luke. It could date from the 50s-60s. 

5. John's Gospel

i) Patristic evidence may indicate that it was written in the 90s, during the reign of Domitian. However, that interpretation may be dubious. Cf. J. A. T. Robinson, Redating the New Testament (Westminster 1976),  256-58. 

ii) The epilogue to John's Gospel (Jn 21) supplies a terminus ad quem for the composition of the Gospel. It was either occasioned by the death of Peter or John (the "Beloved Disciple"). Scholars typically opt for John's death (or the "Beloved Disciple"), but if we accept the internal and external evidence for Johannine authorship, then by process of elimination, Peter's death is a better candidate. That's challenged on the grounds of third-person narration. However, illeism, as well as alternation between first-person and third-person narration, is a stock convention in ancient historiography. Cf. Rod Elledge, "Illeism in Classical Antiquity", Use of the Third Person for Self-Reference by Jesus and Yahweh: A Study of Illeism in the Bible and Ancient Near Eastern Texts and Its Implications for Christology (T&T Clark 2017), chap. 2. 

Mind you, so long as the Fourth Gospel was authored by the apostle John, or even an eyewitness other than John (assuming the Beloved Disciple and the apostle John are distinct), then the date of the Fourth Gospel is inconsequential. 

In sum, I think all four Gospels were probably written between the 40s-60s. I don't have a bulletproof argument, but historical reconstructions are rarely bulletproof. It's a matter of choosing the best explanation.