Use the adjective ingrained to describe the strong beliefs of someone who has thought that way for a very long time, such as the ingrained patriotism of a 20-year military veteran.

Ingrained comes from the word grain, which in late Middle English referred to a certain kind of dye. This type of dye was indelible — unable to be removed — much like ingrained beliefs that are so firmly fixed that they'll never change.

Definitions of ingrained
  1. adjective
    (used especially of ideas or principles) deeply rooted; firmly fixed or held
    ingrained habits of a lifetime”
    synonyms: deep-rooted, deep-seated, implanted, planted
    constituted, established
    brought about or set up or accepted; especially long established
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