Adding dice to checks in Pathfinder: Rise of the Runelord
You must play all cards that will modify the check before making the check.

According to the rules (starting on page 11), each check is broken into a number of steps. In order (emphasis mine):


  
  Determine Which Die You're Using.
  Determine the Difficulty.
  Play Cards and Use Powers That Affect the Check (Optional).
  Assemble Your Dice.
  Attempt the Roll.
  Take Damage, If Necessary.
  


Step three is where you would discard your Blessing of the Gods in order to add the die. If you make it to step five, where you make the roll, it is now too late to play any cards or powers to modify have the check is made.