From Ancestry.com Wiki
Religious directories began as books containing directions for the order of public or private worship. As time passed, some denominational directories began to include lists of clergy and/or churches. Eventually the listings outweighed the direction, and these books became directories in the modern sense. Some denominational directories have dropped all statements of creed, such as The Official Catholic Directory, first published in 1817. Religious directories may also be called registers, annuals, and yearbooks. The information they include differs from denomination to denomination, the minimum amount being the name and address of the church and its pastor.
The information contained in religious directories is significant in two ways. If an ancestor was a clergyman, such directories can guide you to his places of service. This is especially true in the case of itinerant ministers; religious directories narrow down these ministers’ assigned working areas. Also, as with funeral and cemetery directories, religious directories may suggest where to find the contemporary records of the church where the ancestor worshipped.
How to Use Religious Directories
Suppose the ancestor was a Baptist in Rochester, New York, in the first half of the nineteenth century. The American Baptist Register for 1852 shows four Baptist churches in Rochester, the two earliest having been established in 1818 and 1834.22 We know the ancestor lived in Rochester before 1834, so he probably belonged to the First Baptist Church. A check of Baptist churches in modern Rochester shows that the First Baptist Church still exists. We write to its pastor, requesting a check in early records for mention of our ancestor.
In another hypothetical example, the ancestor was a Baptist minister named Henry Smith who lived somewhere in New York around 1850. A check of all Baptist associations in New York revealed only one Henry Smith, who preached at Hastings, Westchester County. With this information we don’t need to check all of the Henry Smiths in the 1850 census index of New York; we can zero in on the relevant one.