A Wrinkle In Time is a bit of fantasy, a bit of science fiction, written for older children and younger teens. Adult readers will find it a bit light, although many have fond memories of reading the book when they were younger. A significant work of literature, it has it's own Wikipedia and Spark Notes entries, and has won the Newbery Medal, the Sequoyah Book Award, and the Lewis Carroll Shelf Award. In addition to these honors, author Madeleine L'Engle held a dozen honorary degrees and received awards such as the ALAN Award for outstanding contribution to adolescent literature, from the National Council of Teachers of English.
I read this book looking for objectionable material, and I found absolutely none. The usual things that provoke censorious ire, such as descriptions of sexual activity, adult situations, strong language, glorification of crime, etc., are completely absent.
Yet the book is challenged. It occupies position number 23 on the American Library Association's list of the 100 most challenged books of the 1990s. Challenges are described in Foerstel's book Banned in the U.S.A.: A Reference Guide to Book Censorship in Schools and Public Libraries and in McClellan's Madeleine L'Engle: Banned, Challenged and Censored.
Objections to the work are primarily religious, at least some of them the result of judging the book on the basis of excerpts rather than reading the entire work. A Wrinkle in Time is frankly religious, asserting the existence of God and a clear demarcation between good and evil. More subtly, the book is also distinctly Judeo-Christian, naming Jesus and quoting scripture (without the usual citation of chapter and verse). Some objections arise because thee of the characters appear to be witches, although reading the whole book reveals that those are merely transient appearances of beings that are later identified as guardian angels. There are no examples of magic or spells, but it is a question of interpretation as to whether some characters are using technological devices or supernatural powers, such as using a crystal ball to show several people what is going on in other parts of the galaxy.
Those who object in spite of having read the entire book are usually fundamentalist or otherwise doctrinaire Christians who object to the liberal Christian, even pantheistic, style of the book (and of the author). For example, some feel that the book demotes Jesus from a unique incarnation of divinity to just one of many teachers. In Trojan Horse: How the New Age Movement Infiltrates the Church, authors Scott and Smith claim that L'Engle's influence is penetrating mainstream Christianity in spite of her denial of basic religious tenets. In Battle to Destroy Truth, Claris van Kuiken called L'Engle's books "repulsive, dangerous, subversive, and treacherous," a characterization that most readers of the book would find incomprehensible. (I'm quoting and paraphrasing these sources from McClellan's book, mentioned above.)
Some parents have chosen to opt their children out of classroom assignments that require reading this book. Disturbingly, that has not been enough for some, who have also called for removing the book altogether from curricula or even library shelves. There is, of course, no legal basis at all for censoring a book over differences in religious perspective.