In colloquial English we tend to think of holy and sacred as being vaguely synonymous, but to the ancestors they were two distinct, if related, forms of being.
The original meaning of holy—Old English hâlig—emerges when we examine its sister-words deriving from the same Old Germanic root: hale, healthy, whole, hail, wholesome, hallow. Holy denotes an intrinsic state of being characterized by radical completeness in self: wholeness, entirety, unbrokenness.
The first observation to make about sacred, on the other hand, is that it derives from Latin rather than Old English. Possibly Latin sacer replaced Old English wîh (or wêoh) because of the latter's pagan associations. If so, they don't seem to have had this problem on the Continent, where the old Germanic word still survives in the Modern German name for Christmas Eve: Weihnacht, “holy night.” (It's worth noting that modern German-speaking pagans refer to Yule as Weihenacht, an archaic form of the same word.)
But in fact both the Latin and Old English words refer to the same concept. What is sacer or wîh is something that belongs to a god. Hence, to sacrifice (literally, “make sacer”) something is to give it to a god. Sacrilege is the theft of something that belongs to a god: in the eyes of the ancestors, one of the most terrible of crimes.