Hebrews 3:7-11 (ESV)
Therefore, as the Holy Spirit says,
"Today, if you hear his voice,
 do not harden your hearts as in the rebellion,
on the day of testing in the wilderness,
 where your fathers put me to the test
and saw my works  for forty years.
Therefore I was provoked with that generation,
and said, 'They always go astray in their heart;
they have not known my ways.'
 As I swore in my wrath,
'They shall not enter my rest.' "
Today, if you hear his voice, do not harden your hearts as in the rebellion.” The one aspect of Lutheran doctrine that people seem to have the hardest time accepting is that it is not your choice to believe, but it is your choice to disbelieve. They think, well if I can choose to disbelieve, then I must be able to choose to believe. It must in some way be my choice to believe, at least partly. I have not chosen to disbelieve. But it is like life itself. We don’t choose to live, (existentialism aside) we just live. We wake up in the morning, we didn’t really choose to wake up. We might choose to go back to sleep. We were awakened by something else. Why else would I be up at 4:15? Riddle me that?! We don’t choose to be born. If we drown at the beach, we don’t choose to be resuscitated. We are simply alive. It wasn’t our choice. We can choose, though it is very inadvisable to treat God’s gift of life this way, to end it. We can choose to blow our brains out. But as long as we are alive, it really wasn’t our choice. We don’t choose to breath. Breathing should never be that conscious an effort. “If you hear his voice, don’t harden your hearts.”
Hardening our hearts, that is the choice of man. We can harden our hearts to the gospel. We can’t choose to believe, God brings us to life. But we can throw the gospel away, say it is too good to be true. Or too suspicious. We can rebel against it. But should we believe it, all credit goes to the work of the Holy Spirit. It is one sided, but so is life.