'one' is used here as a pronoun, meaning 'somebody'. It is not so commonly used these days.Often one uses 'you' instead. ;^). Note that they have in fact mixed pronouns - It should be "If one is to understand the great mystery one must study all the aspects..." 'Is' is used rhetorically. The sentence means much the same as 'If you want to understand...' - or 'If somebody wants to understand'... I don't know what you mean by 'what time'. You can use it in any imagined or planned situation... 'If one was to go to South America, one should visit Macchu Picchu'.