"The great danger of the church lies in her camp followers or hangers-on: they infect the true Israel. Hence the need of guarding the entrance of the Church, and keeping up discipline within it."(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
You can also listen to an audio recording of The Forgotten Spurgeon on Sermonaudio.com: