Sin is so serious because every sin is against God. You can use the following illustration to get this across.
If I took a swing at my friend, there would be consequences but we wouldn’t even make the news. Taking the same swing at a police officer, however, might get me into the papers – and into jail. Same swing – different consequences -because of whom I swung at. Take it a step further. Say I took the same swing at the President of the United States. That would get me national attention – and serious jail time. Same swing – different consequences – because of whom I swung at.
Sin is so serious because it is against God. After committing adultery, David cried out to the Lord: “Against thee, thee only, have I sinned” (Psalm 51:4). This is striking because not only had he committed adultery with Bathsheba; he also had her husband killed. David said this, however, because every sin breaks God’s law and thus is against God. All sin is extremely serious. There are no misdemeanors when it comes to sin. In fact, sin is worse than a felony –every sin is a capital crime. “The wages of sin is death” (Romans 6:23).
Depending on how they react, you could continue by talking about the second half of the verse, the good news: “but the gift of God is eternal life through Jesus Christ our Lord.” Many Mormons, however, are like the rich young man Jesus encountered. They need to wrestle with the law before they are ready to hear the gospel.