Killing someone and missing Sunday Mass are both mortal sins, punishable by eternity in hell. This seems to make the two sins equivalent. But in my mind killing is far worse than missing mass. Are they really equivalent?
No, they are not equivalent. There are degrees to mortal sin just like there are degrees to venial sin. First‐degree murder is more grave than missing Mass, or viewing pornography, or any other grave sin that we might imagine.
It is true that killing someone and missing Mass are in the same category of "mortal," (or grave) sin. But they are no more equivalent, than a rat is equivalent to or the same as a man, simply because they are in the same category "mammal."
Your description of both sins as being punishable by eternity in Hell also implies an equivalence by that fact. However, a distinction is necessary regarding the way you connect the notion of eternity to punishment. That one is in Hell eternally, is not due to punishment per se. Rather, the eternity of Hell (or heaven for that matter) exists because, at some point, our decision for or against God, and the laws and values of his Kingdom becomes a decision that is for us at death forever fixed. Thus, that Hell is eternal, is not by itself a gauge of the punishment involved.
We need not presume that everyone experiences Hell in exactly the same way, any more than we presume that everyone experiences heaven in exactly the same way. There may in fact be degrees of suffering in Hell, and degrees of glory in Heaven.
While there are mysteries involved here, it makes sense that there are some Saints who, on account of extraordinary virtue, have a greater capacity to appreciate God's glory in heaven. It also makes sense that for those in Hell who have rejected God, and his Kingdom, there would be degrees of suffering experienced, related to how deep their rejection of the light is. Scripture indicates we are judged according to what we have done (Revelation 20:11‐15). Thus, there is at least implied some relationship of reward or punishment rooted in what we have done or not done. Jesus also speaks of places of special honor in heaven, indicating levels of some sort in the afterlife (cf Matt 20:23).
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