In the West, the Catholic Church was the majority Christian religion from the eleventh to the early sixteenth century. Its tenets varied a little from place to place, but by and large, people were expected to know/believe:
- what the Trinity was
- what the basic prayers (Lord's Prayer, Creed, Ave Maria) were
- to confess regularly in preparation for mass
- to know they should attend Mass regularly
- that, during mass, the bread and wine literally became the body and bloody of Christ
- to know the seven sins, the sacraments, the mercies and commandments
In the first half of the sixteenth century, there were basically two schools of Protestants. People like Martin Luther believed that the Catholic Church's emphasis on traditional learning and rules built up over time by holy people was too dominant. He insisted thaat people were saved by their faith andby God's mercy, not by the prayers of saints or by good works. His views are very strongly based in the New Testament.
The other Protestant school was Calvinism, which strongly denied the Catholic idea that the bread and wine of the mass became the body and blood of Christ.
There were also many people who wanted to change Catholicism from within - as there had been right from the start. Some of these wanted the Bible to be much more accessible to ordinary people, and they had this in common with the early Protestant reformers.
This is a really simplistic, basic summary, btw!
Anyway, in England, the Catholic Church ceased to be the state religion after the reign of Mary I, and Elizabeth brought in the C of E. There is some disagreement whether this is truly a 'Protestant' religion, as this denomination claims to go back to pre-Catholic roots of Christianity (so do many others, of course). This denomination is a very Protestant-heavy 'compromise' between Catholicism and Protestantism as it was then understood.
IMO all the roots of difference in England go back to this time period.
These days, Protestants differ hugely in what they believe. I would say what defines Christians is that they believe in the Trinity, but whereas Catholics believe that God is literally there in the bread and wine, C of E people all disagree with each other, and most other Protestant denominations believe the ritual is only symbolic. Catholics and the C of E put a lot of reliance on extra-Biblical material, but some denominations would say you can only rely on what is in the Bible.
Socially, big differences are marriage after divorce, sex before marriage, contraception, abortion, homosexuality and women priests. The Catholic Church is wavering on some of these, having historically considered them unforgiveable sins. The C of E is beginning to waver on some (there are women priests, you can be married after divorce, contraception is fine, others are more up to your vicar). I don't know so much about other denominations, but the Quakers are eager for women priests and gay marriage, while some Baptist Churches are very down on all of it.