It may have escaped your notice that 10th July marked the 500th anniversary of John Calvin's birthday. Following from the highly successful 'WhyNotSmile Guide To John Wesley', we will now learn more about the man dubbed many things, including 'Father of Calvinism'.
John Calvin was born in France at about the time of the Reformation, and was Roman Catholic by birth. At some point he had a "sudden conversion" and became Protestant, and therefore had to leg it to Geneva before they killed him.
In Geneva, he started thinking about Things, and in 1536 he published a book called The Institutes Of The Christian Religion; the book was republished several times, and finally weighed in at 4 books of eighty chapters, which is more 'Books You Imagine You Ought To Have A Basic Feel For' rather than 'Rollicking Great Read', and which one imagines may well have benefitted from the fact that there weren't all that many options for Those Who Were Into Reading in those days. Anyway, those who have more persistence with such things than me have given it good reports.
When your average theologian thinks of John Calvin, he thinks immediately of Predestination. This can be summarised as the belief that, before creating the world, God made a list of everyone who would ever live, and then decided which ones He would save. This is probably not how it should be summarised, of course; I'm just saying it can be. Anyway, this is still a point of contention among Protestants today, particularly when expressed in the form of 'double predestination', which says that God also decided who He was going to send to Hell, but lots of people do not think this is correct. As far as I can tell, this also has something to do with the disctinction between 'Protestant' and 'Reformed' churches, but I am not sure what.
There is, of course, More To It Than That; it will not surprise you to hear that Calvin's 'Institutes' (as it is commonly known) dealt with all manner of topics theological, but I herein summarise the main points for your convenience:
1. Total depravity. The idea behind this is that we are all so bad that we would not naturally turn towards God, and are therefore enslaved to sin unless God lifts us out of it. Generally most Protestant churches agree with this, but the Roman Catholic church considers it heresy (and on reading some Protestant writers on the topic, one can be inclined to the impression that this is a major point in its favour).
2. Unconditional election. This bit is all about who receives God's mercy. The 'unconditional' bit means that there are no conditions based on anything inherent within the person concerned. The 'election' means 'choosing', so 'unconditional election' means that God chooses who will be saved based entirely on His own decision and not on anything that that person has done (since, as we have seen in point (1), no one is capable of doing good without God anyway). Anyway, this ties in with predestination, and is violently disagreed with by lots of non-Calvinists (including Methodists, who believe something more in line with 'conditional election', and Catholics, who beleive something else that I'm not at all sure about). Of course, no one on earth knows who the elect are, which gives us all hope.
3. Limited atonement. Ah, Penal Substitutionary Atonement, how many bytes of webspace have been devoted to thy debate. But we call it 'limited' here, mainly so the 'TULIP' thing works. The point is that Christ's death on the cross was for the 'elect' (see point 2), and not everyone (see Catholicism, Methodism, Lutheran etc.).
4. Irresistable grace. This means that once God has His eye on you, you can't turn away. It's a bit like watching a car crash, only God is watching, and has in fact caused the car crash, and it's not really a car crash because it's a Good Thing. Anyway, Methodists disagree, and so do some other people.
5. Perseverance of the saints. In other words, 'once a saint, always a saint'. But if you stop being a saint, then you were never really a saint in the first place. Also, by 'saints' we mean the Protestant and (dare I say it? heck, why not, most of you have stopped reading by now anyway) biblical meaning of the term, as in 'Christian, believer, sinner saved by grace', and not the Catholic meaning, of 'dead person who is now in heaven'.
There are also lots of other things that Calvin either started, or talked about more than anyone else, including ditching all the sacraments except baptism and communion, not being at all keen on the Pope, liking Covenent Theology, and doing all sorts of unexpected things to church structure. But since I mainly don't really understand them, we shall not dwell further upon them here.
Anyway, Calvin's legacy is mostly the Presbyterian Church, and, by some argument which I can't follow, Capitalism.
 As with all WhyNotSmile Guides, the research behind this article is based entirely on the first thing that comes up on typing the subject into Google, plus anything WhyNotSmile happens to remember while typing.
 I have no idea if this is true, as I do not know any average theologians. I have merely introduced it as a literary device.
 The five points listed here are known as the 'Five Points Of Calvinism', or 'TULIP' for short.
 I think. But I'm not entirely sure. This may mean something completely different.
 These are, of course, generalisations. There are probably Methodists who believe the moon is made of green cheese and we're all descended from jellyfish, but I'm saying that this is the official line of the church.