How can we be sure that Jesus' promises to return are false prophecy?
Jesus, it is claimed, prophesied that certain unspecified individuals would not die until they would see either "the Son of Man coming in his kingdom" (Matthew 16:28), or "see the kingdom of God after it has come with power" (Mark 9:1), or see the "kingdom of God" (Luke 9:27).
Jesus, addressing his disciples "privately" (Mark 13:3, Matthew 24:3) listed what was going to happen before his return. He then added, "Then shall they deliver you up to be afflicted and shall kill you and you shall be hated of all nations for my name's sake" (Matthew 24:9). Concerning this Mark's version adds, "he that shall endure to the end, the same shall be saved" (Mark 13:13). Thus, it appears from this last remark that at least some of the disciples would survive and be present to witness the second coming and the end time.
The Synoptic Gospels while in basic agreement on the wording of the first part of their respective verses differ on the wording of the second part. Mark 9:1 is the most informative. It reads in the full literal verse: "Truly I say to you, there are some of those who are standing here who will not taste death at all until they see the kingdom of God having come [that is, after it has come] with power." Apparently, the early Christian community was convinced of the imminent return of Jesus, as the Messiah, and the inauguration of the kingdom of God. It never happened.
Matthew's Jesus makes an explicit statement as to his expectation of returning within the lifetime of many of his contemporaries. He declares, "But when they persecute you in this city, flee into the NEXT: for truly I say to you, you shall not have gone through the cities of Israel, until [heos, that is, "up to the time"] the Son of Man comes" (Matthew 10:23). From this verse we see that Matthew's Jesus promised to return before the apostles visited all the cities of Israel. Jesus commissions the twelve apostles to be missionaries to the Jewish people. The author of Matthew comments, "These twelve Jesus sent forth, and commanded them, saying 'Do not go into the way of the Gentiles, and into any city of the Samaritans do not enter. But go rather to the lost sheep of the house of Israel. And as you go, preach, saying, The kingdom of heaven is at hand'" (Matthew 10:5-7). Here Jesus commands the twelve apostles to proclaim it exclusively to the Jewish people. Jesus then tells the apostles that "the Son of Man" is scheduled to arrive before they accomplish the visitation of all "the cities of Israel." This is the third time the immediacy declaration "the kingdom of heaven is at hand" is used in the Gospel of Matthew. The first time was by John the Baptist (Matthew 3:2). Then, Jesus used it as his opening proclamation (Matthew 4:17). The imminent coming of the Son of Man to usher in the kingdom of heaven is expressed as being "at hand." "Is at hand," is one word, engiken. It has the meaning of "near," in relation to either place or time.
The forthcoming return of Jesus, the Son of Man, is to come rapidly as promised. This never occurred. Moreover, fulfillment of Matthew 10:23 would have had to occur, at the latest, by the time the last of the original band of apostles died. That event has long since past. Indeed, all of Jesus' contemporaries are dead (Mark 9:1) and his promise remains unfulfilled.