Preston, R. H. (1989). Twenty Years after Populorum Progressio: An Appraisal of Pope John Paul's Commemorative Encyclical. Theology, 92(750), 519-525. doi:10.1177/0040571X8909200609
Παραπομπή σε μορφή Chicago (17η εκδ.)Preston, Ronald H. "Twenty Years After Populorum Progressio: An Appraisal of Pope John Paul's Commemorative Encyclical." Theology 92, no. 750 (1989): 519-525, https://doi.org/10.1177/0040571X8909200609.
Παραπομπή σε μορφή MLA (9th εκδ.)Preston, Ronald H. "Twenty Years After Populorum Progressio: An Appraisal of Pope John Paul's Commemorative Encyclical." Theology, vol. 92, no. 750, 1989, pp. 519-525, https://doi.org/10.1177/0040571X8909200609.
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