The following verses point out the first century understanding of 'whole world' or 'oikoumene':
Acts 11:28 (KJV) - And there stood up one of them named Agabus, and signified by the Spirit that there should be great dearth throughout all the world: which came to pass in the days of Claudius Caesar.
Luke 2:1 (KJV) - And it came to pass in those days, that there went out a decree from Caesar Augustus that all the world should be taxed.
2 Chronicles 36:23 (KJV) - Thus saith Cyrus king of Persia, All the kingdoms of the earth hath the LORD God of heaven given me; and he hath charged me to build him an house in Jerusalem, which is in Judah. Who is there among you of all his people? The LORD his God be with him, and let him go up.
Acts 2:5 (KJV) - And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven.
Also, here's some quotes by the early Church Fathers on the gospel having been preached to the 'whole world':
CLEMENT OF ROME (A Contemporary of St. Paul - Late 1st Century)
(on the martyrdom of Peter and Paul)
"But not to dwell upon ancient examples, let us come to the most recent spiritual heroes. Let us take the noble examples furnished in our own generation. Through envy and jealousy, the greatest and most righteous pillars of the Church have been persecuted and put to death. Let us set before our eyes the illustrious apostles. Peter, through unrighteous envy, endured not one or two, but numerous labours and when he had at length suffered martyrdom, departed to the place of glory due to him. Owing to envy, Paul also obtained the reward of patient endurance, after being seven times thrown into captivity, compelled to flee, and stoned. After preaching both in the east and west, he gained the illustrious reputation due to his faith, HAVING TAUGHT RIGHTEOUSNESS TO THE WHOLE WORLD, and come to the extreme limit of the west, and suffered martyrdom under the prefects. Thus was he removed from the world, and went into the holy place, having proved himself a striking example of patience." (Chap. V.-- The First Epistle to the Corinthians)
EUSEBIUS (325)
"THUS, under the influence of heavenly power, and with the divine co-operation, the doctrine of the Saviour, like the rays of the sun, quickly illumined the whole world; and straightway, in accordance with the divine Scriptures, the voice of the inspired evangelists and apostles went forth through all the earth, and their words to the end of the world. (Ecclesiastical History, Book II, Ch.III.)
"But the holy apostles and disciples of our Saviour, being scattered over the whole world, Thomas, according to tradition, received Parthia as his alloted region; Andrew received Scythia, and John, Asia, where, after continuing for some time, he died at Ephesus. Peter appears to have preached through Pontus, Galatia, Bythinia, Cappapodica and Asia, to the Jews that were scattered abroad" (Ecclesiastical History, Book III, Ch. 1)
"The same historian records another fact still more wonderful than this. He says that a certain oracle was found in their sacred writings which declared that at that time a certain person should go forth from their country to rule the world. He himself understood that this was fulfilled in Vespasian. But Vespasian did not rule the whole world, but only that part of it which was subject to the Romans. With better right could it be applied to Christ; to whom it was said by the Father, "Ask of me, and I will give thee the heathen for thine inheritance, and the ends of the earth for thy possession." At that very time, indeed, the voice of his holy apostles "went throughout all the earth, and their words to the end of the world." (Ecclesiastical History, Book III, Ch. 8)
CHRYSOSTOM (375)
"After this again, what is more grievous than all, they shall not have so much as the consolation from love. Then indicating, that these things will in no degree harm the noble and the firm, He saith, Fear not, neither be troubled. For if ye show forth the patience that becomes you, the dangers will not prevail over you. And it is a plain proof of this, that the word shall surely be preached everywhere in the world, so much shall ye be above the things that alarm you. For, that they may not say, how then shall we live? He said more, Ye shall both live and preach everywhere. Therefore He added moreover, "And this gospel shall be preached in the whole world for a witness to all nations, and then shall the end come," of the downfall of Jerusalem.
"For in proof that He meant this, and that before the taking of Jerusalem the gospel was preached, hear what Paul saith, "Their sound went into all the earth;" and again, "The gospel which was preached to every creature which is under Heaven."(9) And seest thou him running from Jerusalem unto Spain? And if one took so large a portion, consider what the rest also wrought. For writing to others also, Paul again saith con-coming the gospel, that "it is bringing forth fruit, and growing up in every creature which is under Heaven."(10)
"But what meaneth, "For a witness to all nations?" Forasmuch as though it was everywhere preached, yet it was not everywhere believed. It was for a witness, He saith, to them that were disbelieving, that is, for conviction, for accusation, for a testimony; for they that believed will bear witness against them that believed not, and will condemn them. And for this cause, after the gospel is preached in every part of the world, Jerusalem is destroyed, that they may not have so much as a shadow of an excuse for their perverseness. For they that saw His power shine throughout every place, and in an instant take the world captive, what excuse could they then have for continuing in the same perverseness? For in proof that it was everywhere preached at that time, hear what Paul saith, "of the gospel which was preached to every creature which is
