(I believe this is relevant for your discipline as well.)
Sample literature review:
From: Purdue OWL Manuscript Writing Style:
Literature Review
Use the past tense to communicate that the research being reviewed has been completed:
In their study of declining congregations, Hoge and Roozen (1979) found that institutional factors were also important.
It is possible to mix tenses if it helps to explain the finding:
In their study of declining congregations, Hoge and Roozen (1979) found that institutional factors may also help to explain congregational decline.
