The best way to write a thesis statement is to base it on a list of brainstormed ideas from the text which answers your prompt question.

Ask: "How did religion affect Frank McCourt's childhood and adolescence?"

As you create a list of text-based answers (and use quotes wherever possible!) you should begin to see certain patterns develop.  Attempt to come up with answers that fit into different categories.  For a typical essay, three different categories is ideal.  These categories may or may not be part of your thesis.  My preference is to write a two-part thesis.  The first part includes the text and author and is a generality of what you want to say.  The second part includes the three categories.

For example:

In Angela's Ashes, Frank McCourt's Irish Catholic heritage strongly influenced his childhood and adolescence in both positive and negative ways.  Because of his religion, McCourt experienced ___________, ____________, and ___________ [insert 3 categories here].

I'm not usually in favor of using both "positive" and "negative" in the same essay, but in this text, you could easily make a case for both.  His religion was a source of both strife and stability, almost equally.  If you don't like this, it would be just as easy to focus only on the negative sides of "religion" in McCourt's life.