sanctum n.

early 12c. as an adjective, seinte, "holy, divinely inspired, worthy of worship," used before proper names (Sainte Marian Magdalene, etc.), from Old French saint, seinte "holy, pious, devout," from Latin sanctus "holy, consecrated," past participle of sancire "consecrate" (see sacred). It displaced or altered Old English sanct, which is directly from Latin sanctus.

From an adjective prefixed to the name of a canonized person, it came to be used in English by c. 1200 as a noun, "a specific canonized Christian," also "one of the elect, a member of the body of Christ, one consecrated or set apart to the service of God," also in an Old Testament sense "a pre-Christian prophet."

It is attested by late 13c. as "moral or virtuous person, one who is pure or upright in heart and life."

The adjectives also were used as nouns in Late Latin and Old French: "a saint; a holy relic." The Latin word also is the source of Spanish santo, santa, Italian san, etc., and also ultimately the source of the word in most Germanic languages (Old Frisian sankt, Dutch sint, German Sanct).

Perhaps you have imagined that this humility in the saints is a pious illusion at which God smiles. That is a most dangerous error. It is theoretically dangerous, because it makes you identify a virtue (i.e., a perfection) with an illusion (i.e., an imperfection), which must be nonsense. It is practically dangerous because it encourages a man to mistake his first insights into his own corruption for the first beginnings of a halo round his own silly head. No, depend upon it; when the saints say that they—even they—are vile, they are recording truth with scientific accuracy. [C.S. Lewis, "The Problem of Pain," 1940]

early 14c., seintuarie, sentwary, etc., "consecrated place, building set apart for holy worship; holy or sacred object," from Anglo-French sentuarie, Old French saintuaire "sacred relic, holy thing; reliquary, sanctuary," from Late Latin sanctuarium "a sacred place, shrine" (especially the Hebrew Holy of Holies in the temple in Jerusalem; see sanctum), also "a private room;" in Medieval Latin also "a church, cemetery; right of asylum," from Latin sanctus "holy" (see saint (n.)). 

Since the time of Constantine and by medieval Church law, fugitives or debtors enjoyed immunity from arrest and ordinary operations of the law in certain churches, hence its use by mid-14. of churches or other holy places with a view to their inviolability. The transferred sense of "immunity from punishment by virtue of having taken refuge in a church or similar building" is by early 15c., also of the right to such. (Exceptions were made in England in cases of treason and sacrilege.)

The general (non-ecclesiastical) sense of "place of refuge or protection" is attested from 1560s; as "land set aside for wild plants or animals to breed and live" it is recorded by 1879 in reference to the American bison.