How will Israel come to believe in their Messiah in the future?
Israel will believe when the remnant sees the resurrected Lord, Messiah, and King coming in the clouds of glory at the Second Coming. At that moment, after the Tribulation, they will believe in a moment, experiencing the same resurrection power that saved Paul on the Damascus road.
The context explains that God's timetable places the birth of the nation at the end of the Tribulation, which is taught as the Second Coming of Christ. Paul was saved miraculously about 2,000 years earlier on the road to Damascus, like one born prematurely, before the due date of Israel's salvation. In the same way, the nation of Israel—not 100%, but the remnant—will yet be saved. They will still see their resurrected Lord and Messiah and King coming in the clouds of glory, and then they will believe in a moment, experiencing that same kind of resurrection power.