What are the arguments that Acts was written prior to 70 AD? What are the reasons, whether theological or secular, for concluding that the book of Acts was written prior to 70 AD?

Please also view the related question What are the arguments that the book of Acts was written after 70 AD?
J. A. T. Robinson, in Redating the New Testament (1976), lays out a thorough case for a pre-70 date for the publication of Acts.  His main points are also maintained by other conservative scholars, such as D. A. Carson, Douglas Moo, and Leon Morris in their Introduction to the New Testament, and in the Reformed Study Bible. The key points are:

Evidence for authorship by a companion of Paul
No mention of Paul's letters
Language considerations
Abrupt ending
No mention of important events in AD 62–70

A companion of Paul
Robinson argues that internal evidence points to a companion of Paul. First, he points to the passages where the author uses &quot;we&quot; in the narrative:

the style of the 'we' sections of Acts (16.10-17; 20.5-15; 21.1-18; 27.1-28.16) is, as Harnack showed, the style par excellence of the writer of the whole when freely composing in his own hand. There is no real ground for arguing that he is here using a source or travel-diary other than his own.

Against those who argue that this supposed &quot;companion of Paul&quot; made too many theological errors to be credible, Robinson argues that &quot;the discrepancies with Pauline teaching have in my judgment been much exaggerated,&quot; for two reasons.  First, in Acts, Paul deals primarily with non-Christians, while in the epistles he addresses Christians – and Paul's only speech to Christians in Acts (to the elders at Miletus) &quot;contains some remarkable parallels with later Pauline writings.&quot;  Second, the author of Acts presents himself as a layman, a historian, not a theologian, which accounts for a lack of precision in some statements (e.g., Acts 13:39).
No mention of Paul's letters
Against a second-century date, Robinson argues that Luke's failure to mention Paul's epistles, though not &quot;decisive,&quot; is still problematic:

Silence on the very existence of the epistles is, as Kummel says, a formidable objection, amongst many others, to a second-century date. It is unbelievable that a later writer should not have made use of them for his reconstruction or at least alluded to them.

Language
Another evidence that Robinson cites is one of language:

Harnack goes on to adduce numerous positive indications of an early dating of Acts derived from the primitive character of its terminology.

The Reformed Study Bible offers some specifics along these lines:

Certain vocabulary points to an early date.  This vocabulary includes &quot;disciple&quot;; &quot;the first day of the week&quot; (later to become &quot;the Lord's Day,&quot; Rev. 1:10); a reference to &quot;the peoples of Israel&quot; in 4:27 (a term later to include both Jews and Gentiles; Titus 2:14); the early title &quot;Son of Man&quot; (7:56); as well as language about geographical and political details.

Abrupt ending
The &quot;rather lame and unfulfilling&quot; ending of Acts, as Carson et al. call it, is widely cited as evidence for an early date.  Robinson quotes Harnack:

The question is: why does the account stop at this point? As Harnack said,

Throughout eight whole chapters St Luke keeps his readers intensely interested in the progress of the trial of St Paul, simply that he may in the end completely disappoint them — they learn nothing of the final result of the trial! Such a procedure is scarcely less indefensible than that of one who might relate the history of our Lord and close the narrative with his delivery to Pilate, because Jesus had now been brought up to Jerusalem and had made his appearance before the chief magistrate in the capital city!


Carson et al. draw particular attention to Luke's failure to mention Paul's release from prison (if he was indeed released), or his execution a few years later, given that his execution would have made a &quot;fitting parallel&quot; to the execution of James recorded earlier, and a similar ending as the gospel of Luke, in which Jesus is executed.
Surrounding events
But the outcome of Paul's trial is not the only event that is not mentioned by Luke.  Robinson breaks down a series of events that are inexplicably absent.  First, Nero's persecution in AD 64:

There is no hint of the Neronian persecution, which because of its excesses won considerable sympathy for the Christians, as Tacitus says. [Am. 15.44.]

The martyrdom of James, the brother of Jesus, would have fit Luke's narrative well, but it isn't mentioned either:

Nor for that matter is there any hint of the death of James the Lord's brother in 62, which took place at the hands of the Sanhedrin against the authority of Rome. [...] No incident could have served Luke's apologetic purpose better, that it was the Jews not the Romans who were the real enemies of the gospel. Yet there is not a hint of James ever falling foul of the Jewish authorities, unlike his namesake, James the brother of John (Acts 12. ff.)

And, of course, there's no mention of the Jewish revolt that lead to the destruction of the temple in AD 70:

Nor is there any shadow in Acts of the impending Jewish revolt, let alone of the destruction of Jerusalem to bear out the earlier prophecies of the Gospel. When last we hear of them, the representatives of Judaism, alike of church (24.21.; 25.1-5) and state
(25.13-26.32), are living in a condition of courteous, if suspicious, detente with Rome. One could never guess from Acts what was to break within a few years.

Conclusion
Robinson concludes:

The burden of proof would seem to be heavily upon those who would argue that it does come from later, and there is nothing, as far as I can see, in the theology or history of the Gospel or Acts that requires a later date [...]. From the internal evidence of the two books we should therefore conclude (as did Eusebius) that Acts was completed in 62 or soon after, with the Gospel of Luke some time earlier.