If you hear His voice, Do not harden your heart.

As thinking persons, we do not always believe what other people tell us. After Jesus rose from the dead on the third day, Thomas declared that "Unless I see Jesus with my own eyes, and feel his side, I will not believe (John 20:25 paraphrased)."  8 days later, Jesus appeared physically before Thomas, and Thomas believed.

Then Jesus appeared physically before hundreds on several occasions.  And many believed.

But there were still some who saw the risen Christ and still did not believe.  These unbelievers even stoned Stephen to death, and persecuted other believers.

No matter what, even if Jesus were to appear to us physically today, some among us will not believe.

If we refuse to believe because of doubts about God, God will in His own time, resolve our doubts. 

But if we refuse to believe not because of doubts, but because of our stubbornness, then God has only one advice...

"IF YOU HEAR HIS VOICE, DO NOT HARDEN YOUR HEARTS." Heb 4:7