Avebury, Delphi, Ryoanji and Salisbury Cathedral Cloister are sacred places. But are they also gardens? Yes: they are enclosed outdoor spaces; they were designed to be beautiful; they were not made for functional horticulture.
Sun, shadows, water, plants and structures are intrinsic to their design. Ryoanji, you might say, is a dry garden. Yes it is, but moss grows around the famous stones and the play of tree shadows on the gravel is part of the fascination.
According to Wikipedia “Holiness, or sanctity, is the state of being holy or sacred, that is, set apart for the worship or service of gods. It could also mean being set apart to pursue (or to already have achieved) a sacred state or goal, such as Nirvana. It is often ascribed to people, objects, times, or places.” I have one quibble with this definition: the Buddha did not recognize a god and Buddhism is the world religion which has had the most influence on garden design.
In a demythologised sense, if you wish, I believe that sacredness remains a vital concern in garden design. We want to have places which are ‘set apart’ from the everyday world of bustling stress, which fill the soul and solace the flesh. I wish those who plan suburban subdivisions and housing estates had an appreciation of how space can be ‘set apart’ and yet connected.