Again it is approaching my bedtime, so I will allow Cardinal Arinze to explain mortal sin which is, by the way, scriptural.
That servant who knew his master’s will but did not make preparations nor act in accord with his will shall be beaten severely; and the servant who was ignorant of his master’s will but acted in a way deserving of a severe beating shall be beaten only lightly. Much will be required of the person entrusted with much, and still more will be demanded of the person entrusted with more.[Luke 12:47-48]
If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly.
[1 John 5:16]