I wouldn't call it a sequel, but it is still dealing with the Catholic Church.

In this one, Vatican City is threatened by the return of a very old enemy, the Illuminati. Basically (according to the book), the Illuminati was a secret society of scientists, started in the 1600's when science was outlawed by the church, and scientists were persecuted. Now, 400 years later, science has come back to take its revenge.

I've probably made it sound really boring, but its got Robert Langdon in it like Da Vinci Code, and I found it easy to understand and absolutely riveting