Calvinism is Christianity
It may seem a somewhat wry answer, but it depends what you mean by "is". Now, before you think I am becoming impossibly abstruse, let me explain what I mean.
If we take the five TULIP doctrines as the essence of Calvinism, it would be very easy indeed to teach and preach the first two and the last two from scripture. The middle one - limited atonement - takes a bit (perhaps rather a lot) more wrestling with scripture, but it can
be based on the Bible, if not extracted easily from it. (I think this is called eisegesis, as opposed to exegesis.) However, it can be done; and it does follow logically from the other four. That is my first point.
My second point is that many if not most of the great revivals of religion in the 18th and 19th centuries were led and spread by staunch Calvinists. Think of Jonathan Edwards in the States, George Whitefield in the States and Britain, more or less all the non-Wesleyan side of the revival in 18th century Britain, and then on into the 1858-1860 awakening in many parts of Britain. Whatever one makes of the distinctives
of Calvinism, it is hard to deny that there enough truth in it for God to bless and use it in bringing many sinners to repentance and to faith in Christ alone for salvation.
Therefore, I think one can truly say that "Calvinism is Christianity." But that does not necessarily mean that "Calvinism is the whole of Christianity." Following the same two lines of argument, one can easily derive the Four Alls of Methodism
from scripture; and one needs spiritual blindness not to perceive the blessing of God upon Methodism in many lands and many times. Again, multitudes of corrupt sinners, and of self-righteous persons too, have been brought to deep repentance and to trust in Christ as Saviour, Lord and Son of God. So one could equally well say that "Methodism is Christianity" - but it is not the whole
Indeed, over many years I have met, and known at length, a good number of convinced Calvinists, and convinced Arminians, who live lives of deep commitment to our Lord - indisputably truly Christian.
Charles Simeon was a great and influential Anglican minister in 19th century England. I believe one of his comments on this debate was that the truth does not lie somewhere between the two extremes, but in both
extremes. The scriptures do seem to teach both, if one takes their plain meaning. Logically they cannot both be true. We must therefore relinquish either the plain meaning of some scriptures, or the use of human logic in fathoming the truth of God's thoughts and work. Is there mystery in God and God's ways which our minds cannot penetrate? Or is all divine truth reducible to a philosophical system of dogmas with no loose ends?