Antithetical typically describes something that is in direct and unambiguous opposition to another thing. It is often used with to.

// The district's new policy is fundamentally antithetical to the school's values.

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"This proposed village development is too large, too sudden, and too antithetical to the character of our village. It threatens the unique and irreplaceable heritage and biosphere, and with that, the lifestyles of the existing community." — Toby Oliver, The Oxford (England) Mail, 30 Mar. 2025

Antithetical and antithesis come from the Greek verb antitithenai, meaning "to oppose." The oldest sense of antithesis refers to a language pattern that contrasts parallel ideas, as in "action, not words" or "they promised plenty and delivered scarcity," and antithetical originally referred to anything that was marked by such antithesis. For example, you could say that the phrase 'action, not words' is an antithetical construction. It is more common, however, for antithesis to mean "the exact opposite" and for antithetical to mean "directly opposite," as in "an idea antithetical to our stated goals."