I was reading Father Peter M.J. Stravinskas‘ The Catholic Answer Book a few weeks ago and on page 173 I was surprised to read that all religious are required to wear their religious garb as a symbol of their vow of poverty.
I looked up and found in Canon Law that Father Stravinskas is absolutely and clearly correct on this:
Canon 669 §1 As a sign of their consecration and as a witness to poverty, religious are to wear the habit of their institute, determined in accordance with the institute’s own law.
§2 Religious of a clerical institute who do not have a special habit are to wear clerical dress, in accordance with canon 284.
Meaning when you see a nun or a priest not wearing their habit/clericals they are in open disobedience to the Catholic Church and thus in mortal sin.
As the late Pope John Paul II said of the religious garb worn by religious is “keeping God on the streets” of the secular city.
 Article 1. Whether Disobedience is a mortal sin?, The Summa Theologica, Saint Thomas Aquinas.