This refers to ‘brimstone’, nick-name of the former brickmaker and some time poacher who ‘got’ religion with the arrival of Methodism. He gave up thieving in lieu of which he studied the alphabet the better to soak up spiritual correction. But he found reading harder than religious fervour.
‘… a great change had been observed in the brickmaker; and though he was still known in the neighbourhood by his old sobriquet of ‘Brimstone,’ there was nothing he held in so much horror as any farther transactions with that evil-smelling element.’
Source: George Eliot, Adam Bede (Harmondsworth: Penguin, 1985 (1859)), p. 280