Lying is an evil (no matter how small)
Catechism of the Catholic Church:
2485 By its very nature, lying is to be condemned…
It can be said that one can use certain licit “mental reservations”…or “discreet language”.
Now lets say one is in the very serious event of the Nazi at the door… and one can not find those discreet words…and others lives are at stake …and one thus ends up lying. What then? Well of course one should have found the other way. But in our weakness we sin …indeed we commit venial sins daily …hence they are called at times “daily sins” for which we pray daily “forgive us our trespasses”…and certainly ones culpability will likely be even further reduced by the event of the Nazi’s with the guns at your door…so ones venial sin will be reduced to an even smaller venial sin (if there is culpability at all)…which God in his knowing mercy can wipe away as one glances at the Crucifix with prayer after the Nazi’s leave…
God knows that in our seeking to live the varied virtues in our lives we will have venial sins in the process…that we being “saints” …being a new creation in Christ in the state of grace…in charity …living in Christ will still have “daily sins” to pray for. He loves us and continues to purify us with his love. (Not that such is an excuse to not seek to overcome our sins more and more in our lives …nor ought we seek or plan to lie in this case …but rather seek to find the right way)
Now all this does not make it “good” to lie. Or not a sin to lie (be it ever so slight). Nor does the ends in this circumstance “justify” the means of the lie. It remains a lie. A sin. But so too does the intemperance that one exhibited that morning at breakfast with too many eggs or the little anger one expressed at ones spouse that afternoon. If one falls here too…it will be another venial sin. Another weakness. Something that ought not have happened. One ought to have found a good way.
But it is somewhat understandable that it may happen in such events of Nazi’s at the door… even to Christians. To saints.
And there is really no point in trying to argue too much about such…
(PS: perhaps one could argue that there is a different “object” involved-- (hence not be a lie–similar to where taking say food by a starving person may be not “theft” by reason of the object…the definition of theft being something different and the universal destination of goods being in effect etc --see CCC 2408–such is a change in the object --it is a different thing than theft)…but again I leave that to the professional theologians to argue out and to the Magisterium to decide scborromeo.org/ccc/p3s1c1a4.htm#I and scborromeo.org/ccc/p3s2c2a7.htm#II)