As with "courage," the martial virtue to which Mary Shelley here yokes "resolution," this word also evokes the characteristic diction of Milton's Satan. In arising to call his forces to assemble, he marks his agenda:
how we may henceforth most offend
Our enemy, our own loss how repair,
How overcome this dire calamity,
What reinforcement we may gain from hope,
If not what resolution from despair.
When Satan ends the assembly by having his fallen legions endorse his plans to corrupt the garden of Eden, Milton's language emphasizes the force of his resolution.
Thus saying, rose
The Monarch, and prevented all reply;
Prudent, lest from his resolution raised,
Others among the chief might offer now,
Certain to be refused, what first they feared.