This is a great question. Since we are dealing with a hundred years, it is possible to address may elements of change. So, I will mention one of the greatest changes that has taken place.
It is safe to say that Christianity is the largest religion in the world (2.1 billion). So, when we speak of demographic change within Christianity, then we are talking about large scale changes. In view of this, we can say that Christianity has sifted in the last hundred years. What was once a very European religion is now no longer. Let me offer some facts.
There are more Presbyterian churches in South Korea that in Scotland, where Presbyterianism was started. China, once a closed country to Christianity, has more Christians than Germany, France and England combined. There is also an explosion in Protestant Christianity in Brazil and nations in Africa. Brazil alone has over fifty million Protestant believers. Even in America, the greatest growth in churches can be seen in immigrant churches. In view of these points, one of the great changes from 1900 to 2000 is the change in Christian population.
The church has moved from the West to the East and South. For more information, take a look at Philip Jenkins, a scholar of modern religion; he has written extensively on this topic with excellent scholarship.
The question was in religion, not on only Christianity but all. Now when we talk of religion, it is but natural that people relate himself to his own religion he believes in. Thats not bad but when we need to discuss about religion, most of the superstitious beliefs have gone though we find here and there among a few tribes still living in nature.
A religion is a particular system of belief in one God or gods and the activities that are connected with this system. I have not studied much on religion but what I feel members of every religious community are increasing but there are few who obey and act upon the teaching of it. There are so many religions and people are ignorant of the religion he believes in. Because they are not interested in spirituality or to acquire knowledge in it but to become a member of the larger group. Many religious leaders are working on conversion.
To me, teachings of all religions are same if we devote to find in it and yet we are divided in the name of religion. The scripture of all of them whether it is the Bible or the Koran or the Vedas of the Hindus are having the same knowledge to make us a man not to aspire more and be contented with what we have.
I think sanctity of all religions have lost because we see people have become self-centered, greedy, jealous, ready to kill each other either for women or wealth or power. People have lost their peace, whereas religious man never lose their.
William Blake wrote poems of "Songs of Innocence" and " Songs of Experience" and there we find what was the condition of our religion during his time. In "Dover Beach" and in his other poems, felt pity on losing the value of religion. W. B. Yeats wrote the poem "The Second Coming" realizing from core of his heart that it is time for God to send another Jesus or some of his son for every religion to purify the teaching at per with the present situation.