Whoever receives his testimony sets his seal to this, that God is true. John 3:33
If we were to discuss the greatest classical composers of all time, we would have different opinions, because we each have different tastes and different backgrounds.
For example, many Russians would say Tchaikovsky, Germans would say Bach or Beethoven, Brits might say Elgar, and Americans might go for Gershwin.
One person says, “I think Gershwin is the greatest American composer ever.” And another says, “I disagree. Aaron Copland is far better.” The joy and freedom of that is mature people ought to be able to share opinions without offense.
But suppose you were talking with a friend about your home town. If you were to say what you have seen and heard there, and your friend (who had never been there) did not believe you, then you are going to be offended. Why? Because he (or she) would be calling you a liar.
That’s how it is if we refuse to believe what Jesus has seen and heard. He has come from above, from the place none of us has been. To disbelieve his testimony is to call God a liar. It is to impugn his character, and nobody wants to be in that position with Almighty God! John Calvin says, “We cannot offer a bigger insult to God than not to believe his Gospel.”
So the opposite of belief is to set yourself against God and refuse the grace he offers you in Christ. This is surely why, in verse 36, we read: “Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him” (3:36).
Do you see now why unbelief is such an insult to God?