Religion lessons do not figure in the Main Lessons curriculum (there is a specific religion lessons curriculum - but to my knowledge, only a handful of schools in this country or in the UK use it) and the Old Testament, Stories of the Saints, the Mahabahrata, or, Greek or Norse Mythologies, are not taught "as religions". Rather, they are 1) taught because at that particular point in the child's development, something in her soul resonates with these particular stories and 2) because we become better human beings when we are exposed to the many facets of the human journey. Thus, the rise of Islam and the life of the Prophet Muhammed as well as stories from the Buddha (and usually, in this country, Native American legends) are also taught.
A key element of Waldorf education is that there is a totality of the journey of humanity which can be both traced and understood and that this large journey is mirrored in the individual journey of each human being. This is not hierarchical in terms of value - more recent cultures are not seen as "better" - only in that they have a different relationship to the human being's core struggle toward ever greater freedom. Thus European cultures are not valued more highly than, say, Japanese or Maori cultures - but what is emphasized is the greater impulse toward human freedom. Again, the human journey takes many roads - Steiner was mainly concerned with the road toward individual freedom. This is not to deny or denigrate the great gifts and achievements of other cultures than the ones he focuses on.
In third grade, children are separating somewhat from their parents, their individuality is starting to express itself more strongly, they are able to accept the concept of rules, of the Law. This is why the Old Testament stories are taught - to let the children identify within their souls with the struggles that the Israelites had with accepting the Law. As any parent of a 9 year old knows, there is a lot of truth here! Later, the morality - and growing lack of it - of the Norse gods works on the soul of the 10 year old as he struggles to find his place in the world and to know right from wrong - the Fall of the Norse gods, the end of their place in the the heavens, can be very meaningful for the child who is shedding his earlier phase of childhood, leaving the spiritual worlds behind, and facing his own individuality. The sagas of the Indian, Persian, Egyptian and finally, Greek gods further explores the human theme of "what is my place on this earth? what is my relationship to the spiritual worlds?" As the 11 year old approaches adolescence, these questions, usually dealt with in the most shallow way in our society, are of great importance and are fed by these myths.
Sixth and seventh grade brings the Middle Ages on through the expansion of Europe into other parts of the world. The 12 and 13 year olds are finding a new relationship to the world - first comes the pragmatic 12 year old whose body is becoming denser and whose very musculature is changing - time to study the Romans, those most practical and pragmatic people ever! The 13 year old is striking out into the world, exploring, finding out where new worlds lie. Time to study the Renaissance as well as the relationships between peoples - time to study Muhammed and the way the Europeans and the Muslims met - and not only fought, but exchanged great ideas and brought huge change to the world. Time to study India and China - the depth and breadth of these cultures - so that when history rolls round to the European incursions here, one has a true picture of what this meant.
So, in Waldorf schools, history is seen as the story of human kind and thus has both mythic and "real" elements to it. I can't go into all the details here - it's just too involved and is the basis of much of anthroposophy - but suffice to say that one might also ask oneself the question "what is real and what is myth?". Anthroposophy is based on spiritual science - thus some of its facts may not be seen as such by others. I suggest one read Christoph Lindenberg's "Teaching History: Suggested Themes for the Curriculum in Waldorf Schools" for an excellent introduction to this topic.
Nothing in the Waldorf curric is ever arbitrary, is ever without good reason.
Homeschoolers are, of course, free to chop and change and not use it - but if one has even slightly more than a passing interest in this form of education, it behooves one to do some study and find out why certain things are taught when they are. Then if one decides to skip something, one has some sense of its purpose in the curriculum and might choose to find substitute material. Having said that, I have been asked, for instance, if I could think of material with which to replace the Old Testament stories and I really can't. I can't think of a similar struggle in a culture which was presented with the Word, with the Law, in this way. Islam, of course, is also centered on a book, on the Word - but the way it was spread and the way it was received amongst the people was quite different, so I don't see a parallel.
I always encourage people to explore how they might work with Waldorf to suit their own religious/spiritual paths. If Waldorf schools can be found in Egypt, Japan, Israel, India, and Thailand, then surely we homeschoolers can also find ways to work with this form of education alongside our beliefs - and without diluting things so as to make them meaningless!