Pharisee and Tax collector

In the parable of the tax collector and the pharisee, we see God's mercy on the tax collector but not the other. Why? This tax collector had probably done more recognizable wicked things than the pharisee, but why does God still forgive him? Even if the pharisee had kept the sabbath and done all the things he said, he still had the worst sin left. Unbelief. We never think of it that way do we? Unbelief is impossible to be made up for, we can't do enough to cover it up.