Why did one thief believe and the other did not?

The distinction between the two thieves on the cross underscores the sovereign will of God in salvation. One thief witnessed the humility and power of Jesus, resulting in a transformation of heart that led to faith, while the other continued to mock. This difference is not attributed to the will of man but rather to God's electing grace, as detailed in Romans 9:11, which emphasizes that election is not based on individual merit or actions. God's mercy is evident as He grants faith to some while allowing others to persist in their sin, illustrating the depths of His sovereign grace.