§1. When the judge questions witnesses legitimately, they must tell the truth.
§2. Without prejudice to the prescript of can. 1550, §2, n. 2, the following are exempted from the obligation to respond: 1/ clerics regarding what has been made known to them by reason of sacred ministry; civil officials, physicians, midwives, advocates, notaries, and others bound by professional secrecy even by reason of having given advice, regarding those matters subject to this secrecy; 2/ those who fear that from their own testimony ill repute, dangerous hardships, or other grave evils will befall them, their spouses, or persons related to them by consanguinity or affinity. Art. 1.
Source: Code of Canon Law (Vatican.va)