Above is a snippet from one book record in the search results of "early church atonement." This book by Junius Johnson was one of the top results. Now you know that Atonement -- History of doctrines -- Early church, ca. 30-600 is the designated language for this topic and a click on this subject string (in blue) will reveal all of the other books on this same topic.
Double-click on the video for full screen viewing. -- Atla Religion Database is the single most important index of theological literature. This is a brief but excellent, fast-moving tutorial from Gonzaga University. Search by scripture passage is highlighted. Stop watching when he begins referencing the Gonzaga web site.
Google Scholar (GS)
Consider Google Scholar as another essential research tool. Its fundamental advantage over other databases is Google's phenomenal relevance algorithm applied across the complete full text of the sources included. Most other databases consist of only of "records" describing a book or article at various levels.
Scholar also has a "cited by" feature that is a useful indicator of how important a work is in the scholarly conversation on your topic.
Tip! Connect Google Scholar with the library's full text access and interlibrary loan services. -- See instructions.