The problem with the “lying to Nazis to save Jews” scenario is not that a venial lie is told to save lives, but that this extreme scenario is used to justify all the convenient lies that people tell in their daily lives.
No one is suggesting such a thing.
The Magisterium does teach that lying is intrinsically evil, it is wrong by its very nature. When people claim that any intrinsically evil act (lying, contraception, other popular sins) is sometimes moral, or becomes moral by a good intention or a dire circumstance, the implication is that all intrinsically evil acts could be similarly justified.
No one here is implying that all intrinsically evil acts can be justified if done for the right reason. The argument is that it is not immoral to deceive an unjust aggressor to prevent harm to an innocent person. The aggressor is the one who is outside the bounds of morality. People do have a right to be free from unjust aggression.
So when someone claims that a venial lie is not a sin at all, due to intention or circumstances, it implies that every intrinsically evil act can be justified in the same way. And that is just not true.
Then let me ask you this; suppose you are walking down the street and an obviously pregnant woman asks you if you know the whereabouts of the local abortion clinic. You happen to know for a fact that the clinic that has been in the middle of the heated abortion debate is right around the corner. What do you do?
According to you, we simply cannot justify any attempt for the preservation of human life, even at its most vulnerable, so we must therefore speak the truth and tell her the location of the clinic. Did our actions further Justice? Was it an act of righteousness that we did not “lie?” Brother JR, a credentialed theologian, once said that there arises a time when we must “let the greater good prevail.” I happen to believe that such a circumstance would be one of those times.
Do you really think it better to let the Nazis arrest, detain and murder an entire innocent family - children, parents and grandparents - than to seek a path of Justice and help preserve their right of life?
Also, there is also a school of thought in the Church that does allow for deliberate falsehoods when circumstances necessitate it. From the Catholic encyclopedia:
“If a man is hid in your house, and his life is sought by murderers, and they come and ask you whether he is in the house, you may say that you know where he is, but will not tell: you may not deny that he is there. The Scholastics, while accepting the teaching of St. Augustine on the absolute and intrinsic malice of a lie, modified his teaching on the point which we are discussing. It is interesting to read what St. Raymund of Pennafort wrote on the subject in his Summa, published before the middle of the thirteenth century. He says that most doctors agree with St. Augustine, but others say that one should tell a lie in such cases. Then he gives his own opinion, speaking with hesitation and under correction. The owner of the house where the man lies concealed, on being asked whether he is there, should as far as possible say nothing. If silence would be equivalent to betrayal of the secret, then he should turn the question aside by asking another — How should I know? — or something of that sort. Or, says St. Raymund, he may make use of an expression with a double meaning, an equivocation such as: Non est hic, id est, Non comedit hic — or something like that. An infinite number of examples induced him to permit such equivocations, he says. Jacob, Esau, Abraham, Jehu, and the Archangel Gabriel made use of them. Or, he adds,
you may say simply that the owner of the house ought to deny that the man is there, and, if his conscience tells him that this is the proper answer to give, then he will not go against his conscience, and so he will not sin. Nor is this direction contrary to what Augustine teaches, for if he gives that answer he will not lie, for he will not speak against his mind (Summa, lib. I, De Mendacious).”
