Sin and Forgiveness in Christianity and Islam

In this book we will try to answer questions like; What is the origin of sins according to Christians? And what is the gravest sin for them? 

How can sins be forgiven? What are the negative results that emanate from their misleading concepts of sin and forgiveness? Is Christianity the only way to salvation? 

What is Islam's notion of the origin of sins? 

And what is the gravest sin according to Islam? 

What are the various ways that lead to forgiveness?