I read and interact with Calvinist with some frequency. I regularly listen to the podcast of Dr. James White, who is a Calvinist apologist. I disagree with White on many issues but I do respect his rigor and love for the text of Scripture.
B.B. Warfield stated, “the world should realize with increased clearness that Evangelicalism stands or falls with Calvinism.” Calvinist do seem to be convinced that they are the very center of soteriological orthodoxy. Warfield even concluded that Evangelicalism cannot stand without it. Of course, I disagree and so do many.
Many? Yes! By many I mean around 1.8 billion of Trinitarians alone. For example, the following groups do not believe in Calvinism.
Roman Catholics (worldwide): 1.5 billion
Orthodox (Greek, Russian, etc.): 225 million
Wesleyan/Methodist: 10 million
Assemblies of God: 48 million
Calvary Chapel: 2-5 million
Church of Nazarene: 2 million
This is not an exhaustive list because it does not include Oneness. Groups that do hold, almost exclusively, to Calvinism (even then some are not) are:
Baptists All (worldwide): 110 million
Anglican (worldwide): 70 million
Lutheran (worldwide): 66 million
Reformed Presbyterian: 2.5 million (US)
Around 300 million Protestants are Calvinist and the estimated remaining 1.8 billion are not. Those who are Calvinist then make up a very small percentage of Protestant Christianity. Talmadge French, a Oneness scholar, has noted that "preliminary research suggests that the actual size of Oneness Pentecostalism is approaching 20 million." (French) That figure is probably closer to 30 million today and it can safely be said that all 30 million reject Calvinism as well.
Although an appeal to large numbers doesn’t make something true or false it does help one gain perspective. Calvinism is a minority view within Protestantism itself. Often criticism is laid against Oneness theology, which is Arminian for the most part, for it being a minority or small. If one has paid attention to the growth of Oneness Pentecostalism you will know that it is rapidly growing but is small in proportion to a group like Catholicism. Large numbers do not make something true or false. If large numbers meant something was true then Catholicism would be true and neither Calvinist or Arminian would agree on that position.