Future fulfillment of the law (and prophets)
In Mt. 5:17 Jesus says the time of fulfillment has come; much of the law and prophets is now being fulfilled in Jesus and his new kingdom. Then, in 5:18, Jesus adds that there will be still more fulfillment in the future: every prophecy found in the law (and prophets) will finally be fulfilled–will come to pass–when the time comes for heaven and earth to pass away in the end. Not even the smallest letter or mark (not a “jot” or “tittle”) of the written (prophecies of the) law will pass away until all comes to pass in the end.
When heaven and earth pass away is when all comes to pass. All the smallest details of the scriptures that prophesy the future and the final end of history–when heaven and earth pass away–will then be fulfilled and pass away. For all the prophets and the law prophesied about the future (see Mt. 11:13), the future that has now come with Jesus and the future that will all come to pass at the end of history.
In Mt. 24:34 Jesus uses some of the same phrases as in 5:18: “truly I say to you;” “will not pass away;” “until all comes to pass.” It is significant that these phrases in 24:34 refer to events in the (near) future: all these things that come to pass are about the destruction of the temple (and Jerusalem) before that generation passes away. So the similar language in 5:18 also refers to events–prophesied in the law (and prophets)–that will come to pass in the (near and distant) future.
An example of future prophecy from the scriptures that Jesus says (in 24:30) will be fulfilled in the end is Dan. 7:13-14 (the royal son of man will come on the clouds of heaven with power and great glory). In that same context (24:29-30), Jesus alludes to writings of other former prophets when he describes the final passing away of the heavens and the mourning of all the tribes of the earth. Thus the sign of his coming (24:30) will include the passing away of those heavens and that (evil) earth–and the fulfillment of those details in the scriptures.
Jesus will also warn disciples not to be deceived by false Messiahs or false prophets (who give false hopes and promise the imminent end of the evil age) (24:4,11). Their pious promises and holy wars will not bring the end (24:6). In 24:24 Jesus predicts that false Messiahs and false prophets will “give” great signs and wonders, an allusion to Deut. 13:1-3 (in the law of Moses).
All (the law and prophets) will finally come to pass (and pass away) when the old heaven and old earth pass away (Mt. 5:18; compare Rev. 21:1); yet much (of the law and prophets) is already coming to pass (and passing away) with Jesus (5:17). Because much is already coming to pass (being fulfilled) with Jesus and his new kingdom, much of the law and prophets is now passing away.