What does John 3:12 mean?

Because Jesus is God, and was with God in the beginning (John 1:1), He has direct knowledge of what He's claiming. Jesus states that if a person won't believe His words about things on earth, they won't believe His words about things in heaven. There are two applications of this. First, some spiritual ideas are fairly easy, and others are not. If a person won't accept the basics of Christianity, such as salvation, they aren't going to grasp more complex ideas, such as the Trinity. This is why some debates over spiritual issues need to be kept between mature believers.

Second, if a person is so skeptical of Christianity that they won't even believe the mundane aspects of it, they have no hope of understanding the gospel. It is one thing for a non-believer to doubt miracles or the existence of God. It's another to be so hardened against God that even the non-spiritual ideas are resisted. This is most easily seen in people who give grossly inaccurate claims about the Bible as reasons why they don't believe. If they won't accept simple, everyday facts, they certainly won't accept miraculous ones.