What is the most fundamental principle or center of the doctrines of grace, known as "Calvinism," by friends and foes alike? B.B. Warfield answered this question in his article, "Calvinism" (see Works, vol. V, pp. 353-69; online here), and his answer may surprise some.
It is not, as commonly supposed, the beloved (and berated!) doctrine of predestination, but the monergism of salvation. Or, the "I" in the familiar acrostic "TULIP." Warfield explains:
Thus it comes about that the doctrine of monergistic regeneration - or as it was phrased by the older theologians, of 'irresistible grace' or 'effectual calling' - is the hinge of the Calvinistic soteriology [doctrine of salvation], and lies much more deeply embedded in the system than the doctrine of predestination itself which is popularly looked upon as its hall-mark.Warfield concludes and summarizes this point in this clear and heart-warming sentence:
Indeed, the soteriological significance of predestination to the Calvinist consists in the safeguard it affords to monergistic regeneration - to purely supernatural salvation.
What lies at the heart of his soteriology is the absolute exclusion of the creaturely element in the initiation of the saving process, that so the pure grace of God may be magnified. Only so could he express his sense of man's complete dependence as sinner on the free mercy of a saving God; or extrude the evil leaven of Synergism (q.v.) by which, as he clearly sees, God is robbed of His glory and man is encouraged to think that he owes to some power, some act of choice, some initiative of his own, his participation in that salvation which is in reality all of grace. There is accordingly nothing against which Calvinism sets its face with more firmness than every form and degree of autosoterism (pp. 359-60).
He who knows that it is God who has chosen him and not he who has chosen God, and that he owes his entire salvation in all its processes and in every one of its stages to this choice of God, would be an ingrate indeed if he gave not the glory of his salvation solely to the inexplicable elective love of God (p. 360).Firmly set against "autosoterism," the Calvinist rejoices, "... by grace you have been saved!"
NB, for a practical expansion on this point into Christian life and worship, please see Ian Hamilton, "Heart-warming Calvinism." For one of the best recent treatments of regeneration, see John Piper, Finally Alive!