"McCheyne somewhere says, 'Depend upon it; it is God's word, not man's comment on God's Word, which converts souls.' I have frequently observed that this is the case. A discourse has been the means of conviction or of decision, but usually upon close inquiry I have found that the real instrument was a scripture quoted by the preacher." (Source: Spurgeon's Sermon Notes)
Recommended reading:
The Conversion of Charles Spurgeon
The Forgotten Spurgeon, by Iain Murray
A Defense of Calvinism - by Charles Spurgeon
The Forgotten Spurgeon, by Iain Murray
A Defense of Calvinism - by Charles Spurgeon
