The Greeks, of whom Paul was very fond, knew quite well that the earth was not flat. Paul also was very educated and fierce in his condemnation of the Christians (at least he was before his vision). Christianity was never intended to replace the House of Israel, he maintained, for he knew that at the conclusion of the great battle of Armageddon, the Jews would be converted (see Zechariah 12-14) when Jesus/Yahweh returned and his feet would stand on the Mount of Olives. The Jews, seeing the wounds in his hands/feet, would instantly realize his identity and go into a national state of mourning.
The early Christian authorities also knew that this salvation would NOT come in the days of the Roman siege, but would occur after the destruction and dispersal of the Jews and after their return to the lands of their fathers in the future. This return is still in progress and the great battle of Armageddon (Gog and Magog) still in the future. There also are traditions recorded by Origen, who writes: "Clement, the disciple of the Apostles, recalls those whom the Greeks designate as antichthonians, (dwellers on the other side of the earth), and other parts of the earth's sphere (or circuit) which cannot be reached by anyone from our regions, and from which none of the inhabitants dwelling there is able to get to us."
This is often how we know, or surmise, what was known and taught by the early saints -- we find out by what is passed on. Clement was a good friend and disciple of Peter, the chief apostle. In trying to find out everything that was known to them, the early church fathers felt they could preserve the church in as pure a form as possible. Thus, there's a wealth of information that tells us that the apostles did not expect the imminent return of Jesus, and that they didn't believe the earth was flat. They knew about the apostasy, the destruction of the temple and they understood it was to be rebuilt in the last days and that the Jews would ultimately accept Jesus Christ.