What is that mortal sin?
Things like idolatry, which Rome says do die in means there is not hope, which those in 2 Mac. 12 died because of, yet RCs invoke it for support for praying for the dead.
2Ma 12:40 Now under the coats of every one that was slain they found things consecrated to the idols of the Jamnites, which is forbidden the Jews by the law. Then every man saw that this was the cause wherefore they were slain.
Which requires RCA to engage to once again argue from silence when trying to support traditions of men, saying they may have repented at the last.