"No person appears less likely to be saved than your religious unbelievers. They wear and armor of proof. You cannot tell them anything new or striking, their heads are helmeted with religious knowledge; you cannot touch their hearts, for they wear the breast-plate of gospel-hardening." - from Spurgeon's Sermon Notes
Further recommended reading & listening:
The Best of The Daily Spurgeon
The Forgotten Spurgeon, by Iain Murray
A Defense of Calvinism - by Charles Spurgeon