Read "Rosa Mystica" at The Eldritch Dark:
Given that Clark Ashton Smith (CAS) was not a very religious person, I read this poem as having no connection to the Catholic title associated with Mary, the mother of Jesus. Rather, I think CAS is using the "Mystical Rose" as a symbol for the fantastic wonders that always seem to be just out of reach, but which can be touched through imagination.
The closing sestet is really quite beautiful:
On orient isles or isles hesperian,
Through mystic days ere mortal time began,
It flowered above the ever-flowering foam;
Or, legendless, in lands of yesteryear,
It flamed among the violets—near, how near
To unenchanted fields and hills of home!
CAS' verse so often appeals to the power of the imagination. For me as a reader, I take strength from this articulation of the strength (and the hope) to be gained from embracing one's own creative forces.