In Tess of the d'Urbervilles, what are thimble-riggers?

You might be familiar with the game where a small ball or pea is hidden under one of three cups or thimbles. You have to guess which cup has the ball after the agile thimble-rigger swiftly moves the cups around. The old name of the game is thimblerig, but you may know it as the shell game.

No matter what you call it, the game is a swindle — you're set up to lose. The thimblerigger is so adept that the term has also come to mean swindler or cheater.

In Tess of the d'Urbervilles, Reverend Clare is thus described:

He was a man not merely religious, but devout; a firm believer — not as the phrase is now elusively construed by theological thimble-riggers in the Church and out of it, but in the old and ardent sense of the Evangelical school . . .