Jerry Adler writes in Newsweek:

[Carl] Sagan was fascinated by the phenomenon that educated adults, with the wonders of science manifest all around them, could cling to beliefs based on the unverifiable testimony of observers dead for 2000 years. "You're so smart, why do you believe in God?" he once exclaimed to [cleric Joan Brown] Campbell. She found this a surprising question from someone who had no trouble accepting the existence of black holes, which no one has ever observed. "You're so smart, why don't you believe in God?" she answered....

Sagan never wavered in his agnosticism.

"There was no deathbed conversion," [his wife Ann] Druyan says. "No appeals to God, no hope for an afterlife, no pretending that he and I, who had been inseparable for 20 years, were not saying goodbye forever."

Didn't he want to believe? she was asked.

"Carl never wanted to believe," she replies fiercely. "He wanted to know."