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.