All Elements are Units or Nilpotent in a Ring with a Unique Prime Ideal

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I'm struggling on the basic verification that in a commutative ring with only one prime ideal $P$, all elements are units or nilpotent. I have all the facts in front of me: The nilradical is just $P$, which is also the Jacobson radical, and if $x \notin P$, then $P + (x) = (1)$--but I can't figure out how to put them together. How do I conclude that $x$ is a unit from this?