Indeed, here is the only way to understand what Christ meant by “this generation.” He specified on many occasions the generation to which He referred as a “generation of vipers” (Matthew:3:7).
These are not pleasant terms and obviously describe sinful mankind in all its generations. We can only conclude, therefore, that Christ is indicating (contrary to the expectation of a last-days great revival or of a Christian takeover of the world) that the human race as a whole (except for the few who believe) will remain in unbelief and rebellion against God until the very end.
There is another variation of this interpretation that agrees with Scripture. Inasmuch as Christ was speaking to Israel, we can also conclude that His words had a special application to the Jews. He was saying that, although some Jews would believe in Him and thus be part of the church, Israel as a whole would remain in unbelief and rebellion until all was fulfilled. Thus Zechariah prophesied that Israel as a whole would remain a “faithless generation” (Mark:9:19).