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 comparatively easy, others are not. If a person won't accept the basic teachings of Christianity, such as salvation, they aren't going to grasp more complex ideas, such as the Trinity. This is why debates over certain spiritual issues are best kept between mature believers.

Second, if a person is so skeptical of Christianity that they won't even believe mundane facts connected to 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 religion that even non-spiritual ideas are resisted. That's not even true skepticism; it's cynicism and a refusal to believe. This is reflected in grossly inaccurate claims about the Bible, often used as reasons why someone does not 'believe. If they won't accept simple, everyday facts, they certainly won't accept miraculous ones.