The sentence is not grammatically correct, it should be, "Last summer, Jim mowed one lawn, now he mows three." Mowed just one lawn doesn't make sense in one sentence.
I believe what he meant was that his belief in god was dead and that god to him was dead. Just like how many Jews were dying so was their hopes and beliefs due to their circumstances they were facing at the time.
So then you could be able to predict what would or could happen in the future.
