APA (7th ed.) Citation

Pius XI, P. (1923). Encyclical letter of H.H. Pope Pius XI (Ubi arcano Dei). London.

Chicago Style (17th ed.) Citation

Pius XI, Pope. Encyclical Letter of H.H. Pope Pius XI (Ubi Arcano Dei). London, 1923.

MLA (8th ed.) Citation

Pius XI, Pope. Encyclical Letter of H.H. Pope Pius XI (Ubi Arcano Dei). London, 1923.

Warning: These citations may not always be 100% accurate.