As John the Baptist's ministry was being the forerunner of Jesus's first coming, Elijah and his
companion will be messengers to prepare the way of the Lord.

............................................This would fulfill Jesus' words after John the Baptist had died, "Elsias truly shall first come, and restore all things". Matthew 17:11

............................................The disciples had just seen Elijah on the Mount of Transfiguration, and they understood that it was the same Elijah who would come and restore Israel to her appointed place.