The simplest explanation would be that Luke wrote his book at the very point of time where the narrative is broken off. Of course he could not tell us any more if nothing more had happened. He brought his narrative right up to date. Nothing more was possible.
It is true, other explanations may be proposed.
(a) It has been suggested, for example, that The Acts closes so abruptly because the author was saving something for another work. As The Acts is the continuation of the Gospel of Luke, so a third work, it is said, was planned as the continuation of The Acts. But even so, it seems rather strange that the author should not have given at least a hint of the outcome of that trial in order to take the edge off our curiosity. He has done something like that at the conclusion of his Gospel; why not also at the conclusion of The Acts?
(b) But perhaps the ending is not so abrupt as it looks. The author's purpose, it is said, was not to write a biography of Paul, but to show how the gospel spread from Jerusalem to Rome. When Rome was reached, then the narrative was broken off. Biographical details—even the most interesting details about the most interesting character—were ruthlessly excluded. The plan of the book had been accomplished. For this explanation there is much to be said. But the trouble with it is that especially in the latter part of the book the author as a matter of fact does show considerable interest in biographical details. The trial and shipwreck of Paul are narrated with a fullness which is quite out of proportion to the rest of the history. After such a full account of the trial, it remains rather strange that the author has said not a word about the outcome.
Either of these last two explanations is perfectly possible. Possibly The Acts was written as late as A. D. 80. But the early date at least explains the peculiar ending best of all.
3. SOURCES
Where did Luke get the materials for his work? Did he use written sources as well as oral information? The question has been discussed at very great length, but without much uniformity in the results. If he used written sources, at least he used them skillfully, placing upon them the imprint of his own style. The book possesses genuine unity.
The really important fact about the sources of the book of The Acts is a negative fact. Whatever the sources were, the Pauline epistles were not among them. Compare the passages where Paul and Luke narrate the same events—for example Gal., chs. 1, 2, with the corresponding passages in The Acts—and it becomes evident that the two narratives are entirely independent. Luke did not use the Pauline epistles in writing his book. That is an exceedingly significant fact. It shows that The Acts is an independent witness. What is more, it strengthens materially the argument for the early date of The Acts. The Pauline epistles at a very early time began to be collected and used generally in the Church. In A. D. 100, for example, they would certainly have been used by anyone who was writing an account of Paul's life. Since, therefore, the book of The Acts does not use them, that book must have been written earlier, and probably very much earlier. Even in A. D. 80, it would perhaps have been strange that the epistles should not have been used.
4. PURPOSE
The proper purpose of a historian is to tell the truth. And Luke was a genuine historian. His own account of his method, Luke 1:1-4, shows that he knew the meaning of historical research, and the character of his books bears this out. Luke did not permit any desire of putting Christianity in a good light, or of defending one kind of Christianity against another, to interfere with the primary duty of truthfulness.