You're probably referring to The Ideological Origins of the American Revolution.
The English Libertarian Heritage, edited by David Jacobson, contains excerpts from Cato's Letters and their other series The Independent Whig. Free downloads of both (unabridged) are available here:
A two-volume hardcover version of Cato's Letters is also available from the same source. I am not aware of any readable and unabridged modern printed editions of The Independent Whig; but the online version is public domain and can be printed and spiral bound by your local print shop for about what a book would cost.
The Independent Whig is a scathing review of the corruption that organized Christianity had degenerated into in both England and on the Continent. The theme was that religion only works as a means of promoting civic morality when it is not used as a power base.
Yes, its the Liberty Fund Vols I & II that I have.