The law shall be known ... that is, the Book of the Law is outwardly revealed.
The half known and half concealed rituals can be interpreted literally on many levels. Looking at 'the rituals' most commonly associated with Thelma, as published by AC in Book 4, and the rituals of the OTO and AA, even if they are fully known by the candidate there will be a certain unknown element in each individual ritual. They OTO rituals (and perhaps the AA) were actually only 'half known' up to recent times, with much research and collation being needed to develop a full system.
Even developing your own rituals from BoL you might think its 'all known' ... that is until you try to enact or perform the ritual ... then see how much of it you 'knew'.
The ordeals ... in relation to the above two; law and ritual .. are written not ... that's the 'surprise factor'. No one can predict what effect any ritual will have on another or yourself, the ritual will be empty unless 'sealed' by an ordeal (although some may say visa versa) ... most usually the case with initiation type rituals.
Yes, our life is full of rituals but certain types of magical ritual are enacted to affirm these often unconcious ritualised processes. The ritual stimulates the unconcious/artistic side while the ordeal 'stimulates' in a physical, concious, emotional and sometimes 'gut-wrentching' way.
Humans are too thick to learn from simple instruction ... like computers we need to have the info punched in. Writing or predicting the ordeals would subdue their impact.
Indeed, true intiation cannot be achieved by taking a title, a fancy costume or jewel and going through the ceremony (and paying for it

) - that comes via the ordeal ... the ceremony in the temple is supposed to help one understand, collate, predict, plan, etc the strange whirlpool of events (that can seem unrelated and confusing) some of us call the ordeals.
A series of intitiation rites is supposed to prepare one for, or to consolidate one after, the ordeals of life through which we all evolve.