The Bible cautions and admonishes against both sinning and "leading others into sin." How to make it easy to understand why "leading others into sin" is extra bad and will incur severe chastisement? I thought of this analogy.
Suppose you are a person who drinks alcohol. So think of the ability to consume a glass of wine or other alcohol beverage as a normal dietary activity, part of your meal, or to be social, even as a healthy thing to do if we believe the "Mediterranean diet." So if your religion does not prohibit alcohol and if you have a moderate amount, let's use that as the baseline in this analogy of being "no sin."
Suppose you have a little too much on one occasion and you drive yourself home, taking a risk that you are "buzz driving," but you make it OK. You can see that this gets into a gray shading area, where it's not the "no sin" of the first example, but you "got away with it." So it's dicey but it was only the one time, so it's "no sin but not exactly smart or noble."
Now let's suppose that you make a habit of drinking and driving. That gray just got a lot darker because you are putting yourself and others regularly at risk. So you've entered into the sin zone.
Now let's suppose that you regularly drink, drive and carry passengers. Whether you actually ever have a wreck or not that is very much in the sin zone. Why? Not just because of the risk but because you are now setting an example to others that this is OK to do AND giving your loved ones reason to worry and be unhappy and scared. So this is very much a strongly sinful stage.
(Remember, I'm using this as an analogy, as you'll see clearly with the next few examples. I'm not making up or assigning new classes of sin for the Vatican to publish!)
Now let's suppose that you drink, drive, carry passengers and also purchase alcohol for minors using your ID and allow minors to drink in your house. You've now made that sin contagious. That is called "leading others into sin." When you "just" had the passengers, that was setting a bad example AND leading them into danger, but not leading them into sin. But when you enable minors, in this example, you are leading them into danger of addiction or harm associated with consumption, plus breaking the law, plus depriving them of good parental authority. It's harmful in a lot of ways but most of all because you put them on a path to be able to sin on their own. That is why "leading others into sin" is huge, and very outrageous in God's eyes. God knows everything through all time and God will continue to "track" all the people who you led into sin. So if the minors develop habits or sin on their own, all of this will be known to God and attributed to you.
Now suppose that you drink, drive, carry passengers, purchase alcohol for minors AND call out to everyone in a bar or restaurant "open bar on me!" Now I'm not using an example of a function such as a party or wedding where you pick up the bill for the event and that includes open bar. I'm using an analogy. Here, though, you create a "free for all" of "leading others into sin." In other words, without knowing the age, disabilities, possible addictions, or whether they will drive themselves home or not, you let the alcohol flow without any limitation. This is like "leading others into sin" by having an open bar tab. And what is this most like? Creating a false religion.
Someone who creates a false religion is like shouting "open bar" at an AA meeting, at children's schools, and among the lonely, and running a tab that goes on for hundreds of years. Why do I say this? Because in this analogy you are offering unrestricted alcohol consumption to those who are the most vulnerable to abuse. People who create their "own churches" basically open a never ending bar tab that will draw customers from the weak and vulnerable who have fallen away from institutional religions, or are too young to have the choice, because their parents bring them to "your bar." Now remember, I'm not talking about mainstream religions, even those that are very local and non-denominational, yet still in mainstream Christian beliefs, for example. I'm talking about "personal revelation" churches, cult of personality churches, financially or sexually exploitative churches, cults, paganism, witchcraft and so forth. When, in this type of analogy, a person founds a cult type or extreme non-denominational church, one is basically creating a bar tab for a substance that goes on as long as there are believers. This is not only "leading others into sin" but putting "a false god before God." And so, in God's eyes, even after you would be dead and judged, the actions of this "open bar tab" live on after you, and God will account them to YOUR tab, and not in a nice way.
I hope that you all find this analogy helpful. Again, don't get all word smithing with me because like I said, it's only an analogy folks, but it's a very important one. It is simple to understand the degrees of personal sin and then, using this analogy, where it steps into "leading others into sin" and finally to idolatry and false gods territory of mortal sin.
Just a theological thought to share. Now that you understand this analogy, you can understand why Martin Luther is such a controversial figure. When people get combative about him, it is the slippery slope of sinning/leading others into sin that each side is really talking about without saying so. The people who side with the Reformation obviously feel that he "saved" people from sin because he "challenged the abuses" and broke away with the Church that was "sinning" and "misleading." However, thoughtful people on the other side correctly wonder about his accountability for "leading others into sin" because of, using this analogy, the "open bar tab" that was created after him and resulted in, let's not lie about it, the breaking of the unity of the Church that is contrary to what God and Jesus wanted. So I'm not taking sides and I'm not dropping hints as to Martin Luther's judgment by God. But I am using him as an example of how every soul must make a wise decision about both personal sin AND leading others into sin. It is not inconceivable that, as the saying goes, "The road to hell is paved with good intentions." Popes have commented, including our current Pope Benedict, that Martin Luther would have been very grieved by much that has been done in his name, for example, the diminishment and hostility toward Mary by some Protestants, for Martin Luther himself loved and revered Mary. (Don't ever discount Mary; she might even have bailed him out of that sticky situation! :-)
Thus, one who thinks they have extraordinary "spiritual insight" must also recognize that if they act outside of the institutional churches, that he or she is opening an "open ended bar tab." Even the best of intentions in doing so may result in the unforeseen tragedy of leading many after you into sin. This is why the Bible and the Qur'an are very filled with warning about those who prophesy falsely or worship false gods. It's more than personal sin, because it becomes that "open bar tab" that goes on and on for centuries, potentially, and results in intentional or unintentional "leading others into sin."