unpardonable sin

She was the one black sheep of the fold, for she had committed the unpardonable sin—she had laughed at Mr. Puffin.

Incidentally, “side” is the unpardonable sin among our colonial brethren, and rightly so.

I suppose they've not neglected in Boston to tell you there is such a thing as 'the unpardonable sin'?

And he supposed away till he lost touch with the world and with what it permits, and committed an unpardonable sin.

Indifference, known by its hard heart and its callous temper, is the only unpardonable sin.