This paper has a threefold argument: First that Christianity more than any other cultural or intellectual force was responsible for the idea that every human being, no matter what their economic or social standing, is of immeasurable worth. Second that the Christian church, as a voluntary and far-flung association of believers, was one of the originators, if not the originator of the concept of government by representation. Third, that the protestant reformation of the 16th century, by powerfully reinforcing the concept of human individuality, also promoted the idea of human equality, and that this in turn led to the rise of modern democracy. By democracy I understand that form of government in which a) all people share political power through elections to a representative governing body and b) all members of the polity possess basic individual rights or civil liberties.
First, what about Christianity? One of the features that differentiates it from the Judaism out of which it grew, is that the latter has a much more collective understanding of human salvation. It is the people, the house, the nation of Israel, over whom God watches, and for whom he has a special concern. In the Old Testament God usually rewards or punishes the nation of Israel corporately. In the New Testament by contrast, salvation becomes a fundamentally individual matter. "Work out your own salvation in fear and trembling" St. Paul admonished the Christians at Philippi.  Salvation is an individual matter, and every person has a value in the eyes of God that is beyond price. Thus Jesus' parable about the shepherd who takes infinite pains to find and rescue the one sheep from his flock that has been lost.  Or again, each one of us is so important to God that "every hair on your head has been counted."  Or recall the depiction of the day of judgement at the end of Matthew's gospel. Each individual will be welcomed into heaven or consigned to hell according to whether he gave food to the hungry, drink to the thirsty, hospitality to the stranger, clothing to the naked, and visited the sick and the prisoner. And who are the hungry, the thirsty, the naked, the stranger, the sick and the prisoner? They are images of Christ himself.  So every human being, even the man in prison, has a value comparable to the Son of God. I think this teaching about the ultimate and immeasurable worth of each human being is more powerfully present in Christianity than in any other system of belief, including secular humanism. Christianity, along with Judaism, tells us that man is made in the image of God.
Secondly, what about the church? The church is the company of believers, the voluntary association of those who accept Christ as their saviour. Since the Church, unlike the state did not possess coercive power over its members, it had to make decisions in a consultative fashion. Any attempt to govern the church autocratically or to force unpopular policies on the membership would simply have resulted in people voting with their feet and leaving. As the church became a far-flung enterprise it became possible for all the members to assemble in one place. So the practice arose of holding councils, at which representatives from different parts of Christendom would gather to make decisions on behalf of all the members of the church. It was at these councils that the various creeds were approved and adopted, that various scriptures were either adopted or rejected as part of the biblical canon. The convening of councils continued from the early centuries through the Middle Ages, and up to the present day. The representative principle was succinctly enunciated by Pope Innocent III at the first Lateran Council in 1215 when he declared "that which affects all should be decided by all". So the church may claim to have originated the principle of representative government. But that is not the same as democracy. Those who attended church councils were exclusively members of the clergy -- they weren't just bishops and cardinals and popes, but they weren't the laity either. The clergy, as the church's spiritual Úlite, were deemed the appropriate ones to make decisions on important questions of doctrine and church government.
So the church was a representative, if not a democratic, institution, from very early times. What about protestantism? The German Augustinian monk Martin Luther, by his defiance of the institution and the rulers of the Catholic church of his day, strongly reaffirmed the Christian idea of the importance of the individual. And he added another explosive ingredient: the assertion of human equality. The clergy were no longer to think of themselves as the spiritual Úlite, rather they were to become the servants of the church, ministers to the laity. No minister or pastor was to consider himself superior to the flock which he tended. The individual's relationship to God was no longer to be mediated by priests, or any other institutional apparatus -- sacraments, holy water, saints, relics, or other holy objects. Luther summed up his idea of equality in the phrase "the priesthood of all believers" and he put it into action by translating the Bible into German to make it accessible to ordinary people, and administering both the bread and the wine at communion to the people. Before Luther only the priests had consumed the wine at communion.
The early protestants were committed proponents of education. Their ideal was universal literacy so that everyone -- women as well as men -- could read the Bible for themselves, and work out their own salvation. The leading protestant reformer in the generation after Luther was the French lawyer and theologian John Calvin. To Calvin titles, ranks, human honours, lineage and blood lines were of little importance. In his system peasants, artisans and merchants were the equals of kings and noblemen. Calvin never tired of emphasizing how often God imparted his gifts of wisdom and insight to the poor and lowly, and denied them to the mighty and the rich. While the conservative Luther retained the hierarchical structure of the church, the radical Calvin favoured the election of ministers by the congregation. In Geneva, which became the first laboratory where Calvin's ideas where put into practice, the city was highly democratic in structure, and the principle of the sovereignty of the people was recognized. Calvinism spread to many parts of Europe, and had its greatest impact in Holland, Scotland and England. Where Calvinism held sway bishops were done away with and the church was governed by national and provincial synods, representative bodies composed of laity as well as clergy, which met on a regular basis, not just every century or so. Calvinism spawned Presbyterianism, and that more radical brand of protestantism known as congregationalism. In the most extreme brand of congregationalism, English separatism, each congregation was its own self-governing republic.
During the English revolution of the 1640s congregation not only chose their own ministers, they also decided for themselves what they would believe. This as you can imagine, led to a great splintering of Christian belief, as people gravitated to the congregation whose doctrines most closely approximated their own. So you had multifarious groups known variously as Brownists, Baptists, Family of Love, Seekers, Fifth Monarchists, Ranters and Quakers, as well as the more orthodox Presbyterians and Congregationalists. Many of these groups encouraged lay people to preach, because they said God was just as likely to impart his revelations to mechanics and tinkers as to learned theologians. Others even authorized women to preach and prophesy. While some historians maintain that protestantism reinforced the bonds of patriarchy in early-modern society, others point out that protestantism, and puritanism in particular, that hotter brand of protestantism, as it was called, proclaimed the spiritual equality of men and women, and urged that marriage should be companionate, and husband and wife soulmates.
It wasn't long before people began applying the lessons and the habits that they had picked up in their radical churches to the political realm. If all were equal in the sight of God, if all decisions were to be made by the congregation by majority vote, why should the state not operate along the same lines. And so you have the rise in London of a mass movement called the Levellers. Many of them were members of radical congregations. They denounced economic and legal privileges, and called for election of MPs on the basis of something very close to manhood suffrage. Furthermore, the constitution of England should be a binding, written covenant, circulated to all the people and deriving its authority from their collective signatures. When in 1649 a group of Leveller women demonstrated before the House of Commons for the release of their imprisoned leader John Lilburne, they were scolded by the Speaker for meddling in politics, and told to go home and tend to their housewifery. Undaunted, they came back the next day with another petition, prefaced with a ringing statement of their equality with men.
Since we are assured of our creation in the image of God, of an interest in Christ equal unto men, as also of a proportionate share in the freedoms of the commonwealth we cannot but wonder and grieve that we should appear so despicable in your eyes as to be thought unworthy to petition or represent our grievances to this honourable House. 
What is perhaps surprising is that none of these women, nor anyone else in the seventeenth or most of the eighteenth century, thought to demand votes for women. Women's suffrage didn't become a live issue until after the middle of the 19th century.
In the new church structures spawned by Calvinism people had been encouraged practice the doctrine of human equality, and to govern themselves according to the principle of one man, one vote. The classical monarchical doctrine that kings were anointed by God and derived their authority exclusively from him, was challenged by the assertion that all power derived from the people, and was entrusted by them to kings. Sovereignty was contractual, and kings who violated their contract with the people by ruling arbitrarily or tyrannically could have their powers revoked by the representatives of the people -- namely by parliament. In fact Calvin, who disapproved of popular rebellion against unjust rulers, did suggest that a tyrant could be called to account by the lesser magistrates in his jurisdiction -- the judges, MPs, and other officials with whom he shared his sovereignty. It was on the basis of this Calvinist political theory that the English parliament in 1649 put King Charles I on trial for treason against his own people, found him guilty and beheaded him.
The conviction that all power derives from the people, who have the authority to call kings to account and to overthrow them of necessary, was systematically developed and given an abstract, secular cast in the writings of the late 17th-century political philosopher John Locke, most notably in his Second Treatise of Government. We should not forget, however, that Locke was born into a puritan family, was a believing Christian -- one of his books is entitled The Reasonableness of Christianity, and that in his writings he assumed the Aristotelian and Catholic doctrine of the natural law. In Locke not only are the people the source of all legitimate power; they possess certain inalienable rights. They have the right to speak freely, to worship God in the way they choose, to associate with whomever they wish, the right not to be imprisoned without cause, and by due process of law, and the right not to be taxed without their consent in parliament. The indispensable foundation of human liberty is the right to property. But note that Locke uses the word in the large, 17th-century sense, meaning propriety, or ownership -- ownership not only of land and money, but of your whole person, and everything that you as a person produce. Thus whatever of the earth's resources that I mix with my own labour becomes mine; I have propriety over them. A key conclusion that Locke derived from his doctrine of propriety was that the people, if their rights are seriously and systematically violated by their rulers, have the right to rebel and overthrow those rulers. Hence, the doctrine that all power stems from the people is taken to the radical conclusion that the people may use violence to overthrow constituted authority whenever they deem it necessary.
The Calvinist idea that rulers can be called to account by lesser magistrates; the congregationalist idea that legitimate authority derives from the people, who accordingly have the right to choose those who shall govern them, and Locke's idea that the people through their propriety over their own persons, are endowed with certain inalienable rights, crossed the Atlantic, where they exercised a profound influence on the thought of the American revolutionaries. They in turn exercised a considerable influence on the French revolutionaries of 1789.
It is interesting and significant that when it came time to frame the Declaration of the Rights of men in July of that year it was the Abp of Bordeaux who stated
this idea [of making a Declaration of Rights], conceived in another hemisphere, ought to be transplanted here in our land... America has shown us upon which principles we ought to rest the preservation of our liberty; and it is the new world where in times past we had shown no zeal for chains, which teaches us today so that we will not bring misfortune upon ourselves. 
In summary then, the modern idea of democracy has its foundation not in the secular humanism of the 18th-century French enlightenment, not in the philosophy of Rousseau and John Stuart Mill. No, its foundation is to be discovered, first in the Christian scriptures, with their emphasis on the preciousness of each human being, second in the practices of the early church where the principles of representative government were developed, and thirdly in the protestant reformation with its assertion of radical human equality, its deduction of inalienable human rights from the tradition of natural law, and its grounding of legitimate political power in the people.
1. Philippians 2:12
2. Luke 15:4-7
3. Matthew 10:30
4. Matthew 25: 34-46
5. The Humble Petition of Divers Well-affected Women (1649), British Library, 669.f.14/27
6. quoted in Kingdon, Calvinism (1970), p. 5.