For me, reading it around 1978 or 1980 was a truly eye-opening experience. Firstly, one aspect I appreciate about L'engle's writing is that she isn't afraid to incorporate more adult themes into her work. Time travel, the past influencing (even erasing) the future, and possible sexual abuse are all present, but not dwelled upon. Instead, they are interesting and challenging concepts that are worthy of reflection. Secondly, I grew up in an evangelical household with a very authoritative view of Christianity, and my parents were sexist and bigoted. Reading about a woman who could be a scientist (even though she works from home) and about Indians who were more mature than the white colonists who feared them were both completely novel perspectives for me. Additionally, L'engle's philosophy, which is always gently present, of a kind and loving deity who creates beautiful music, as well as her portrayal of those with belief as actually open-minded and likeable people (again, a first in my experience), was quite refreshing. Her philosophy of attempting to be part of a greater pattern is not confrontational. There is no epic battle in this book. Charles Wallace saves the world by helping someone be with the person they love, at the expense of himself, but it is done without fanfare, hardly even noticed by anyone. There is no hero fantasy being played out here. Isn't that an interesting idea? Meg saves him by asking for help instead of strapping on a sword and fighting for him - isn't the idea of asking for help instead of being entirely self-sufficient against the standard tropes of fantasy? There is no sidekick-hero dynamic here. Everyone does what they can do, and the least appealing character in the book is ultimately the one who placed herself, before anyone even knew what she was doing, between her family and the powers of darkness.
I'm not a Christian myself, but I have more respect for this sort of philosophy than the fire and brimstone variety. And perhaps submission is an awkward trope to understand, but you certainly don't come across much like it in these days of spunky, sassy heroines who are simultaneously completely sexy and completely virginal in popular teen books. I don't advocate submission as the solution to everything, but maybe butt-kicking isn't the answer to everything either.
Don't misunderstand me, sometimes a good sassy butt-kicking is exactly what I enjoy reading about. But reading about community, hope, and harmony again was a refreshing change.