Is it always a sin to lie? For example, what if the Gestapo asked me if I had seen any Jews? Does the Church teach that I should tell the N*** officer the truth even if it will result in suffering and death?
The Catechism of the Catholic Church’s treatment on lying can be found in paragraphs 2475–2487. In a separate treatment of the subject that falls under the heading “Respect for the Truth” (2488–2492) the Catechism states:
The right to the communication of the truth is not unconditional. … This requires us in concrete situations to judge whether or not it is appropriate to reveal the truth to someone who asks for it. Charity and respect for the truth should dictate the response to every request for information or communication. The good and safety of others, respect for privacy, and the common good are sufficient reasons for being silent about what ought not be known or for making use of a discreet language. … No one is bound to reveal the truth to someone who does not have the right to know it. (CCC 2488–2489)
This means there can be cases in which it is morally licit to reserve the truth. Avoiding giving the location of people in danger of being murdered would be one such case. A person in such a situation should try to avoid directly lying, if possible; however, fear for the safety of innocents and the unjust external pressure brought to bear on him could mitigate against culpability should he inadvertently cross the line to lying.