The timeline above shows the period during which the mountain was known as the 'Halifirien'. Its traditional Elvish name, used among the Gondorians, was Amon Anwar, 'Hill of Awe', but when Calenardhon was settled by the Northmen they gave it a name in their own tongue. It was after this time (Rohan was settled from III 2510) that Amon Anwar became known as the 'Halifirien' ('holy mountain'), and by the end of the Third Age this was its most usual name.
The mountain had long been considered a holy place (hence the original name 'Hill of Awe') but during most of the history of Gondor the reason for this was not generally known: on the mountain was a hallow that held the Tomb of Elendil. After the founding of Rohan, Elendil'stomb was relocated to Minas Tirith, but the Rohirrim maintained the tradition by calling the Halifirien 'holy mountain' in their own tongue.