Hi, Andrew —

Thanks for the question.

You said:

• How can I help a friend who is having doubts about their Christian faith?

I don't mean to sound like a smart alec but the short answer is

By knowing your own faith and sharing it with him.

The key is to develop a personal prayer life and motivating yourself to learn what it means to be a Catholic Christian.  Probably 85% of our Church doesn't know what they should know, so don't worry. 

The Catechism is a great place to start. You can buy it and other good books on my books and periodicals page.

There are three sections on that page:

1. The  AskACatholic.com recommended reading
2. Mike's picks, and
   
3. Eric's picks

You can also read the Catechism on-line. The search engine is at the bottom of the page or you can start with this page which has the search engine at the top of the page.

Mere Christianity, by C.S. Lewis is another great read.

He shows how Christ could have only have been:

• a Liar
• a Lunatic or
• the Lord Himself.
  
  and rules out the first two logically.

Hope this helps,

Mike