“For all the prophets and the law prophesied until John, and if you are willing to receive it, this is Elijah, who is to come.”
Again we have the emphasis on the fact that the new age has come. The prophets and the Law prophesied until John. That is, the whole of the Old Testament Scriptures led up to the time of John because he is the last of the prophets, the Elijah who was due to come. All were therefore in the end preparing for the coming of Jesus. The thought that the prophets and the Law were now achieving their end would have been quite startling to the Jews. To them the prophets and the Law were the basis of all their beliefs (at least theoretically). That somehow Jesus was now achieving what they were pointing to, and capping them off, would have huge significance. He was not destroying the Law or the prophets but fulfilling then (Matthew 5:17).
Note the prophets are unusually mentioned first (contrast Luke 16:16) because the emphasis is on the prophetic movement ending with John, but the Law is included (that is all the books of Moses) because it was an important part of that prophecy. It was indeed the basis and starting point from which the prophets themselves made their pronouncements. And now the long series is seen as having come to an end in John, the promised Elijah. What happens from now on is the fulfilment, as Matthew constantly makes clear.
‘Until John.' The ‘until' may be seen as including or excluding John. But it is doubtful if we can exclude John from being one of the prophets, even though the last and greatest. That would not, however, prevent John being the connecting link between the two ages, issuing out the old, and introducing, in a preliminary way, the new.
What the doubt was about was whether they themselves would believe it, firstly because they were expecting Elijah's literal return (he had not died but had been taken up into Heaven), and secondly because if they did accept it they would have no choice but to recognise in Jesus Himself, the Coming One. And certainly some did believe that John was the intended Elijah (as had been made clear at John's birth), and some did enter under the Kingly Rule of Heaven. And when Paul first arrived in Rome the offer of the Kingly Rule of God was still being made, to both Jew and Gentile, an offer closely connected to their response to Jesus Christ (Acts 28:23-24; Acts 28:31), an offer that was indeed made continually throughout Acts (Matthew 1:3; Matthew 8:12; Matthew 14:22; Matthew 19:8; Matthew 20:25). So there is no way in which it can be said that the idea of the Kingly Rule of Heaven was set aside to await the future.
Some have argued that John could not be the fulfilment of Malachi 4:5 because he was not successful enough, but that is to underestimate John's impact. ‘There went out to him all the country of Judea, and all the people of Jerusalem' (Mark 1:5). Even granted the exaggeration, that is some impact, and it would have been even moreso when people visited Jerusalem at the feasts. The widespread nature of his success comes out in the fact that decades afterwards disciples of John were still found around the Roman world.
Nor is there anywhere any suggestion that Jesus did ever offer the Kingly Rule of Heaven to Israel in a way that could either be accepted or rejected as though it was a whole take it or leave it deal. Right from the start He offered the Kingly Rule of Heaven as being available to those who would respond, knowing full well that they would only be a minority (Matthew 7:13-27). He never expected wholesale acceptance, even though He was grieved that the cities of Galilee that were closest to Him on the whole refused to repent. But that was because of His compassion and because His heart longed for them, not because He was really expecting them all to respond. The only change of tack that He would make was that He would offer it to others because those to whom it was first offered had not on the whole accepted it (Matthew 21:42) (but that was in fact in accordance with His expectations as Matthew 7:13-27 demonstrates).