The 1874 RLDS edition added the definite article the before things, and the RLDS text has continued this reading. However, there is no need to add the the. Elsewhere in the text, there are other occurrences of “things which are” without any preceding determiner, including this one similar to 1 Nephi 22:1:
Summary: Maintain the earliest text in 1 Nephi 22:1, which lacks the before “things which are spiritual”.