The entire book is available from Project Gutenberg, and there is a plot summary in the book's Wikipedia article. I wish to briefly state the major theme.
The major theme of the book is that you cannot truly be yourself, until you are willing to give up yourself. In this, the book is an example of Matthew 10:39 "Whoever finds his life will lose it, and whoever loses his life for my sake will find it." (ESV, Jesus speaking)
MacDonald (and, I think, Jesus) is speaking on two levels here. First, you can't really accomplish anything worthwhile in life without making Jesus Christ Lord of that life. It's tempting to think you can, but you won't. Second, of course, is the heavenly reward for the believer. Two of MacDonald's characters spend most of the book trying to live for self. Mr. Vane (I don't think the name is an accident -- he is unsettled, subject, as it were, to the direction of the wind.) the protagonist, wants to do good, but tries to do it on his own, and the result is that his heart's love dies, and he doesn't accomplish much, if any, of what he had tried to do. Besides that, he has misled a group of children who depended on him for leadership. Lilith (from a legend, Adam's first wife, before Eve) who is still alive after all these years, makes no pretense of doing good. She is almost entirely selfish. But she knows, deep down, that she needs to yield herself to the ministrations of Adam, Eve, and Mara (sorrow) their daughter. Finally, she does.*
There are many plot twists, and I don't think I've given away too much, for anyone who wants to read this book.
*It is likely, as the Wikipedia article on the book suggests, that MacDonald is arguing for Christian universalism -- everyone, even Satan, will eventually be redeemed. MacDonald was not orthodox in this, of course, but, other than that, he is.
Thanks for reading. Read MacDonald. I have quoted from the book a few times in previous posts. To see these, click on "Lilith" in the tags following.