It was a tradition of the Pharisees following the Prophet Malachi, that Elias should come before the coming of the Saviour, and bring back the heart of the fathers to the children, and the children to the fathers, and restore all things to their ancient state. The disciples then consider that this transformation which they had seen in the mount was His coming in glory, and therefore it is said, “And his disciples asked him, saying, How then say the Scribes that Elias must first come? As though they had said, If you have already come in glory, how is it that your forerunner appears not yet? And this they say chiefly because they see that Elias is departed again
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).