Matthew 17:13

Then the disciples understood. It was clear to them that John the Baptist fulfilled the prophecy. It is not to be expected that "Elijah" will come a second time to fulfill a prophecy that has already come true. Elijah could not now restore Judaism, because that system was superseded in Christ and the new covenant. Elijah would not likely be chosen to restore Christianity, if it needed restoration, since a Jewish prophet would be "out of place." One of the apostles would better fit that job,