Sincerity refers to telling the truth, while fidelity refers to being loyal to someone/ not betraying someone.
- Janet wasn’t lying. I could see she was speaking with total sincerity. (I could see she was telling the truth.)
- There can be no doubt of the sincerity of his words. (There is no doubt he was telling the truth.)
- Marital fidelity is not as common these days as it was a hundred years ago. (People cheat on their spouses more often these days.)
- The soldiers swore fidelity to the King. (They swore they will be faithful to the king.)
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