The Four Marks of the Church

Sources
Tour of the Summa https://www.catholictheology.info/
Baltimore Catechism
https://www.catechism.cc/catechisms/Baltimore_Catechism.pdf
The First Vatican Council
https://www.papalencyclicals.net/councils/ecum20.htm
The Sacraments Explained
https://archive.org/details/5thsacramentsexp00deviuoft/5thsacramentsexp00deviuoft/
The Catechism of Trent
https://www.ecatholic2000.com/trentcat/untitled-58.shtml
AD SINARUM GENTEM
https://www.vatican.va/content/pius-xii/en/encyclicals/documents/hf_p-xii_enc_07101954_ad-sinarum-gentem.html