Salvation and The Narrow Gate

Letters of St Catherine of Siena (Noffke) ACMRS Press
The Dialogue of St Catherine (Noffke) Paulist Press
Thomas Aquinas, Commentary on the Gospels of Matthew and John 4 Volume Set (The Aquinas Institute; 1 edition September 12, 2013), Vol I, P159
Pope Pius X Lamentibili Sane (July 3, 1907) http://www.papalencyclicals.net/pius10/p10lamen.htm