Nephi is explaining about the crafty wiles of Satan in 2 Nephi 28:8, where Satan says, "Eat, drink and be merry; nevertheless fear God - - He will justify in committing a little sin..."
I'm remembering when I was a child we visited some cousins on my mom's side. My aunt was drinking coffee and explained that she didn't think God would keep her out of heaven for a cup of coffee. I knew that wasn't right but didn't have the words to disagree.
Nephi explains it perfectly. Good won't keep her out of heaven, but she is choosing to keep herself out of the Celestial Kingdom by choosing her addiction to coffee over obedience to the commandments of the Lord. To be with the Lord eternally in His house requires total dedication and desire to do His will. Of course we will make mistakes and repent but our deep desire and longing is "if I but learn to do His will I'll live with Him once more".