"Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son."
God sent his Son into the world to bear witness on our behalf. The testimony of Jesus Christ is that though we are guilty, we shall be declared not guilty. We are justified and set free from punishment. This is the testimony God has concerning us, that though we have sinned, all is forgiven.
However those who do not believe this message and thus does not trust in the Son of God makes God out to be a liar.
When faced with this beautiful truth, it is unthinkable that anyone should respond by crucifying Christ. A messenger brings good news and he is insulted, beaten, and killed instead of welcomed. They do not believe the gospel of Jesus Christ and so accuse God of being a liar. How broken are we that beautiful truths bring pain when we hear them? Oh how far we've fallen!
As anyone who has gone through an abusive relationship can tell you, after being told lies and being put down all your life, you can hardly believe when people tell you the truth. You cannot believe someone loves you and moreover, it actually hurts when someone speaks to you words of affirmation. Believe in the good news and be patient with those enslaved by the world. God is not a liar.