She gripped my hands, and her face was utterly sincere.

Vyland was quiet, convincing, utterly sincere and could have talked his way past St. Peter.

Though my sister sounded utterly sincere, I wasn't in any mood to hear it.

His voice was utterly sincere, and to Jander's surprise, the elf found himself believing him.

He seemed utterly sincere, but carried along as though against his will.

His voice rose to an utterly sincere hysterical wail.

I thought him to be utterly sincere; a true believer in what he promised.

And she manages to be utterly sincere about her healing activities without veering into proselytizing.

This last might have have been utterly sincere or mildly facetious.

They aren't lying - indeed, they're all trying to be utterly sincere - but they aren't telling the truth.