Word for caring about the appearance of a matter but not its spirit

The word usually used, at least in Christian theology, is legalistic.

Dictionary.com gives a fitting definition of the noun (the noun and adjective have closely corresponding meanings):

legalism noun

• strict adherence, or the principle of strict adherence, to law or prescription, especially to the letter rather than the spirit.

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Lexico is more incisive in its definition, from a Christian perspective:

legalistic adjective

• 1 Adhering excessively to law or formula.

  • ‘our conciliation process avoids an overly legalistic approach’
  • ‘narrow legalistic definitions’

• 1.1 [Theology] Concerning adherence to moral law rather than to personal religious faith....