Purba, Michael Jordan. “Williamson, G.I. Katekismus Heidelberg: Sebuah Panduan Studi”. Diligentia: Journal of Theology and Christian Education 2, no. 1 (January 31, 2020): 120–121. Accessed November 20, 2025. https://ojs.uph.edu/index.php/DIL/article/view/2207.