Matthew 4:8–9 does not mean that Jesus and Satan could see the whole world from the mountaintop, but they had a literally elevated position from which Satan showed his reign over the whole world (John 12:31). However, Christ came to redeem the world from Satan’s power (1 John 3:8), which was already clear early in his ministry (Luke 10:18), and even more just before his ascension (Matthew 28:18).
No, it is important to understand the words in context here. There is, of course, no mountain on earth from where the whole world could be visible, but this is also not what the text says. The text says that Satan showed
Jesus all the kingdoms of the world. In this sense doing so from a mountaintop is not insignificant, and is in fact highly symbolic: the whole world over which Jesus and Satan is contending are at their feet. At that moment the world was in the hands of Satan, as he had been its ruler (John 12:31). Christ came to earth for the very purpose of redeeming the world from the power of Satan (1 John 3:8) and he would of course accomplish that (Luke 10:18). God would give all authority to Christ (Matthew 28:18).1
8 Le diable le transporta encore sur une montagne très élevée, lui montra tous les royaumes du monde et leur gloire,