I am guessing that means things in common to all Christians.
You are likely to have a difficult time understanding this text. Almost by definition discussions of religion will have many mystical or vague aspects to them and those of us who have not read the book will have a difficult time understanding the writer's intention.
If this book is important to you, then I suggest you focus on the big aspects of the meaning and don't try to figure out every single turn of phrase used in it. We won't be able to help you.