Of course all sins are sins, but surely you would agree that there is a difference between, for example, genocide, and being rude to someone in a moment of anger? Isn't one clearer worse than the other, and worthy of more punishment? I can't imagine that God would punish the first sin in the exact same way He would punish the second. Can you?:?
From a human persepctive I would agree with you. But if you will take a moment to try and process with me why, I will try to explain why I actually do not think that in terms of sin there is any significant difference between the two sins you mentioned (or any others).
One of the keys here is my use of the word "significant". Normally, we think of things being more or less significant in terms of being bigger or smaller. So, if we think of sins as some sort of crime, then clearly genocide is a worse crime and one might then suppose a worse sin.
However, not all sins are crimes. And it may even be that not all crimes are sins. (I'm here speaking of American law, I don't know sharia law well enough to comment on that.) So, we should not be to quick to agree with the idea that because something is a bigger crime that it is also a bigger sin.
What is it about something that makes one thing a sin and another not a sin? The answer to that is found in God. Does it measure up to what God's will is? When we violate what God's will is for us, then we are sinning. The very same action by another person may not be a sin, it may in fact be something that God wills for the person to do. For example, God may desire me to show affection to my wife, but even if God will for you to care for my wife, I doubt if his desire is for you to show her affection in the same way that he wills for me to do it.
But sometimes we fall short of doing what God wills for us. That falling short is itself sin. One of the Greek words used in the Bible to talk about sin was
hamartia which means "missing the mark". Now it does not make any difference whether you miss the mark by 1 mm, 1 meter, or 1 km. If you missed the mark you missed the mark. What is significant in determining whether you have sinner or not, is not the amount by which you missed the mark, but whether or not you actually hit the mark. So, sin is missing the mark by any amount. You are only righteous if you hit the mark, everything else falls short. And in a Christian concept of righteousness, falling short by any amount is significant, because scripture tells us that the consequence of sin (any sin, any falling short of God's standard of righteous perfection) is death.
Think of it like this, imagine you have to escape from a burning building, and the only way out is to climb to the roof and there board a helicopter that will take you to safety. You and many others are all in the same predicament. And the message comes to all of you of who you be saved. Some decide that they don't want to climb to the roof, they instead head a different way and get trapped by the fire. Some think the whole thing is a fairy tale and they do nothing, only to be consumed by the fire where they sat. Some like you head for the roof, but lolly-gag around about it and are overtaken by the fire. And then there is you who actually makes the roof and see the helicopter. Now the helicopter can't land on the roof because of TV and radio antenae but it has lowered a ladder down for you to take hold of and once you grab hold it will fly you away to safety. The helicopter has flies as low to the roof as it possiblity can without become ensared in the antennae, but the ladder is still just a little bit out of your reach. You stretch out your hand, and can almost touch the bottom rung with your finger tips, but not quite. Finally just as the roof starts to collapse around you, you make one last attempt to save yourself and jump and fall .1 millimeter short. You plunge back into the fire and die.
Now tell me, what was more signigificant, the direction you went, the obstacles you overcame, the amount you climbed, or the fraction you fell short? That is the way it is with sin. Fall short or fall long, either way, you just fall.
Also, when it comes to forgiveness of our sins or shortcomings, it doesn't isn't any more difficult for God to offer grace and still accept us despite our big sins than it does for him to do so despite our little sins. Certainly, from a human persepctive those of us who are aware of the enormous shortcomings present in our lives in meausring up to what we know God wills for us will experience a greater sense of gratitude because of it. But truly, it is God's grace, not how big or small our sins are that enable us to be saved at all. So there is no point in thinking that I am not as bad as someone else. Without God's grace, we all end up in the same place whether we are talking big sins or little sins, sin is sin and it means spiritual death (and eternal separation from God) in the long run. Unless you can figure out how to have larger and smaller lengths of eternity without God, then the whole idea of larger and smaller punishments just does not apply.
I think you even have this same things with respect to forgiveness in Islam:
Then the Messenger of Allah said, ‘Did you not know that Islam wipes out what came before it, and that Hijrah [Emigration for Allaah's cause] wipes out what came before it and that Hajj [the Pilgrimage performed in Makkah] wipes out what came before it!” (Sahih Muslim).
So it is that it matters not how small or big the sin was in the past, they are all equally wiped out by Allah's unmatched forgiveness. Is that fair? Is it fair that a person who has committed genocide might in submitting to Islam might find the same level of forgiveness that one who was just rude in a moment of anger? Is it fair that through the same process of submitting to Allah they they might each find the same reward though one had done truly horrible things and one that which appears less so. But the real sin lies deeper than either genocide or rudeness is that they had lived lives that were not in submission to Allah. This is what was really changed in become Muslim. And Allah forgives them both that equally. And when they repent of that sin of not submitting and start submitting, then both leave the past behind equally as well. Yes the genocidal murder realized the greater grace that he has experienced, but if the rude person had not also given his life over to Allah and submitted, he would have been just as lost as if he had been a genocidal murderer. So, it seems, whether we are talking Christianity or Islam, a sin is a sin, and the difference is not the size of our sins, but whether we live in our sins or in God's/Allah's will.
Hope that helps.
(BTW, since when did this thread become "Things in Christianity Malaikah is curious about"?

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