What does it mean to say that all believers will be resurrected, but also that those who are alive will Christ returns will meet him in the air? Augustine says that there is some mystery here, but the key thing to remember is:
The manner in which this shall take place we can now only feebly conjecture, and shall understand it only when it comes to pass. For that there shall be a bodily resurrection of the dead when Christ comes to judge quick and dead, we must believe if we would be Christians. But if we are unable perfectly to comprehend the manner in which it shall take place, our faith is not on this account vain. Now, however, we ought, as we formerly promised, to show, as far as seems necessary, what the ancient prophetic books predicted concerning this final judgment of God; and I fancy no great time need be spent in discussing and explaining these predictions, if the reader has been careful to avail himself of the help we have already furnished.Just because we do not perfectly understand the mystery does not mean that the mystery is false. Which would be a wonderful way to conclude a discussion of eschatology, but Augustine has more to say on the subject.