First; there is a class of commentators who regard the coming of the Holy Spirit on the day of Pentecost as the fulfillment of Christ's promise to return. This view is based upon our Lord's Word in John 14 where, after declaring to His disciples that He would give them "Another Comforter" who would abide with them for ever," He immediately added, "I will not leave you comfortless: I will come to you" (vs.18). But to regard the descent of the Holy Spirit at Pentecost as the accomplishment of Christ's promise "I will come again" is to confuse the Persons of the Holy Trinity. A sufficient refutation of this error is found in the fact that the Epistles which were all of them written after Pentecost contain numerous references to and promises concerning the personal return of Christ.
Second; another class of commentators regard the destruction of Jerusalem by the Roman armies in A. D.70 as the fulfillment of our Lord's promise to come back to the earth and, untenable as this theory is, strange to say, it has met with a very wide acceptation among Christian theologians. This theory is based upon a careless exposition of Matt.24. At the beginning of this chapter we learn that His disciples asked our Lord three questions: First, "Tell us, when shall these things be?" The "these things" look back to the previous verse where Christ had foretold the destruction of the temple. Second, "And what shall be the sign of Thy coming? Third, "And of the end of the age?" Now in order to understand our Lord's complete answer to these three questions it is necessary to pay close attention to the parallel passages found in Mark 13 and Luke 21. A careful comparison of these chapters will make plain the different answers which our Lord returned to His disciples' questions. In His answers He made a clear distinction between the destruction of Jerusalem and His subsequent personal return, though we must remember that as "history repeats itself" some of the signs which heralded the approach of each event were common to both. When speaking of the former He said, "When ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh" (Luke 21:20); but when referring to the latter He declared, "And there shall be signs in the sun and in the moon, and in the stars; and upon the earth distress of nations, with perplexity; the sea and the waves roaring; men's hearts failing them with fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken. And then shall they see the Son of Man coming in a cloud with power and great glory. And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh" (Luke 21:25-28)  (see footnote). That the destruction of Jerusalem did not exhaust the predictions made by our Lord with reference to His own return is evident from the fact that in the book of Revelation -- written at least twenty years after the destruction of Jerusalem -- He promises, no less than six times, to "come again."
Third; another class of commentators regard the death of the believer as the fulfillment of our Lord's promise to come back again and receive His own unto Himself. This error has already been refuted in an earlier chapter so that nothing further needs now to be said concerning it.
In Acts 3:18 we have enunciated a principle which supplies a sure and certain key to prophetic interpretation -- "But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, He hath so fulfilled." The important words here are "so fulfilled." How had the Old Testament prophecies concerning the "sufferings" of Christ been "fulfilled?" The answer is literally. And, in like manner, will be accomplished those unfulfilled prophecies which speak of the coming "glory" of Christ. Just as those predictions which made it known that Christ should be sold for "thirty pieces of silver," that His hands and His feet should be "pierced," that He should be given "vinegar, mingled with gall" to drink, -- just as these were fulfilled to the letter, so the Scriptures which declare that he shall "descend from heaven with a shout," that "every eye shall see Him" when He comes back to earth, that He shall return in power and great glory and shall be accompanied by "ten thousands of His saints" -- just so shall these predictions be fulfilled to the very letter.
 Note that in Matthew and Mark it is the Sign of the Coming of the Son of Man and of the End of the Age which is in view; while in Luke 21 down to the middle of vs. 24 it is the destruction of Jerusalem in A.D. 70 and the conditions which preceded that event which are set before us.