Take a deep breath, Receiver. Someone is really serious. Few words are more poignantly suggestive of permanence than a promise. This Bearer assures the Receiver they will keep their word, whatever the cost; only you, however, Receiver, can decide how much faith to place in such assertions.

The doubtful have I don't trust you or You've gotta be kidding me with which to make their distrust clear, or Patience to advise playing a long game; more optimistic Receivers, however, can respond lovingly with I love you or Only you or gratefully with Thank you, and declare intent to demonstrate their gratitude with I'll do anything.