If anyone should ask me what I mean by a Calvinist, I should reply, “He is one who says, Salvation is of the Lord.” I cannot find in Scripture any other doctrine than this. It is the essence of the Bible. “He only is my rock and my salvation.” Tell me anything contrary to this truth, and it will be a heresy; tell me a heresy, and I shall find its essence here, that it has departed from this great, this fundamental, this rock-truth, “God is my rock and my salvation.” What is the heresy of Rome, but the addition of something to the perfect merits of Jesus Christ—the bringing in of the works of the flesh, to assist in our justification? And what is the heresy of Arminianism but the addition of something to the work of the Redeemer? Every heresy, if brought to the touchstone, will discover itself here. I have my own private opinion that there is no such thing as preaching Christ and Him crucified, unless we preach what nowadays is called Calvinism. It is a nickname to call it Calvinism; Calvinism is the gospel, and nothing else. I do not believe we can preach the gospel, if we do not preach justification by faith, without works; nor unless we preach the sovereignty of God in His dispensation of grace; nor unless we exalt the electing, unchangeable, eternal, immutable, conquering love of Jehovah; nor do I think we can preach the gospel, unless we base it upon the special and particular redemption of His elect and chosen people which Christ wrought out upon the cross; nor can I comprehend a gospel which lets saints fall away after they are called, and suffers the children of God to be burned in the fires of damnation after having once believed in Jesus. Such a gospel I abhor.
Taken from A Defense of Calvinism by C.H. Spurgeon