The exact formulation of the formative principle of Calvinism, as I have said, has taxed the acumen of a long line of distinguished thinkers. Many modes of stating it have been proposed. Perhaps after all, however, its simplest statement is the best. It lies then, let me repeat, in a profound apprehension of God in His majesty, with the poignant realization which inevitably accompanies this apprehension, of the relation sustained to God by the creature as such, and particularly by the sinful creature. The Calvinist is the man who has seen God, and who, having seen God in His glory, is filled on the one hand, with a sense of his own unworthiness to stand in God's sight as a creature, and much more as a sinner, and on the other hand, with adoring wonder that nevertheless this God is a God who receives sinners. He who believes in God without reserve and is determined that God shall be God to him, in all his thinking, feeling, willing--in the entire compass of his life activities, intellectual, moral, spiritual--throughout all his individual, social, religious relations--is, by the force of that strictest of all logic which presides over the outworking of principles into thought and life, by the very necessity of the case, a Calvinist. If Predestination isn't the key issue in Calvinism (which Warfield states it isn't)
But rather love and awe of God which makes one a Calvinist, then Arminius and Wesley were among the first-rank of all Calvinists!
No two men held God in higher esteem and awe while seeing the corruption and depravity of man.
It is only when you bring in Predestination and mans ability to choose or not choose to receive or reject God that one now becomes a Non-Calvinist (but still a very good Christian).
So, it is Predestination that is the central issue of Calvinism despite Warfields claim otherwise.