Unhallowed

It is safe for Imric the elf-earl to steal the Dane-Chief Orm's son because the child has not yet been hallowed to any gods. Orm is a Christian, therefore his son is not protected by the Aesir. However, Orm is also an indifferent Christian, therefore the child has not yet been christened. Thus, as the witch advises Imric, "'...hallowed to no gods of any kind.'" (The Broken Sword, p. 20)

In The Broken Sword, as in Anderson's The Merman's Children and in Poul and Karen Anderson's The King Of Ys, there are various supernatural beings and forces and, among them, the Biblical God is acknowledged to be extremely powerful. It is like the difference between an ordinary country and a world super power.

The witch swears by Sathanas and says:

"'I do not fear gods or devils, elves or trolls of men.'" (The Broken Sword, p. 20)

Should that read "...or men"?

"Like a rush of wind and a blur of moonlight [Imric] was out of the woods and across the fields." (ibid.)

Here again, the elf is described as if he merges into or has emerged out of nature. Faerie holdings were as if they:

"...wavered halfway between the mortal world and another..." (p. 21)

The idea of something halfway between worlds maybe leads to the idea of a "Half-world."

"Imric rode toward Elfheugh, which he saw not as a tor but as a castle..." (ibid.)

And one name for a tor incorporates the word "castle." See the linked Wiki article. It is the human imagination that sees tors as elven castles and, if such castles did exist, then they would look most of the time like tors.