MATH PUZZLE: Can you replace...

There's this guy who had been lost and walking in the desert for about 2 weeks. One hot day, he sees the home of a missionary. Tired and weak, he crawls up to the house and collapses on the doorstep. The missionary finds him and nurses him back to health. Feeling better, the man asks the missionary for directions to the nearest town. On his way out the backdoor, he sees this horse. He goes back into the house and asks the missionary, "Could I borrow your horse and give it back when I reach the town?"
The missionary says, "Sure but there is a special thing about this horse. You have to say 'Thank God' to make it go and 'Amen' to make it stop."
Not paying much attetion, the man says, "Sure, ok."
So he gets on the horse and says, "Thank God" and the horse starts walking. Then he says, "Thank God, thank God," and the horse starts trotting. Feeling really brave, the man says, "Thank God, thank God, thank God, thank God, thank God" and the horse just takes off. Pretty soon he sees this cliff coming up and he's doing everything he can to make the horse stop.
"Whoa, stop, hold on!!!!"
Finally he remembers, "Amen!!"
The horse stops 4 inches from the cliff. Then the man leans back in the saddle and says, "Thank God."