Less well-known that it should be is the British Library’s recent venture of making subsets of its collection metadata available for download and reuse on a Creative Commons Public Domain Dedication basis.
Of particular interest is a dataset extract from the British National Bibliography in March 2015 for theology, subdivided into monographs and serials. The BNB extends as far back as 1950, and my count suggests that there are some 119,000 entries in the monograph file, and 4233 for serials. This looks to be a incredibly rich resource for thinking about the discipline in the last few decades. My initial searches suggest that there is a great deal here for ecclesiastical history as well.
The files may be downloaded near the foot of this downloads page.
Update: special care is required in the period before 1960, as there is a very large slump in numbers of monographs included between 1955 and 1959. The data is derived from the Dewey decimal classification (200-300); and there seems to have been a decision taken and then reversed not to classify certain works with those classmarks.
Total numbers of records (5 year segments):