I recently read this again after reading it first many years ago and enjoyed it just as much, even knowing what happens.
Many situations in the book are very amusing and the whole idea is irreverent but never intended in my view to insult any religion.
It is very different and is not what I'd describe as SF exactly although there are some elements there.
Heinlein's later work seems to deal with love and affection between people, decency, fairness and indeed Justice as in the title. From my point of view he deals with these subjects well and in a natural, untwisted way.
Some of thw worlds visited are very funny, especially those most like ours. One constant is that whilst the powerful and almighty appear bent on being as unhelpful as possible, there is a willingness running through the whole story of the ordinary person to offer nothing but help and decency.
It is outrageous and funny and not one to miss.