Analysing data from surveys performed in 1991, 1998 and 2008 in 30 countries, researchers from the National Opinion Research Center of the University of Chicago found that, on average, 43 per cent of those aged 68 and older were certain that God exists.
By comparison, an average of 23 per cent of people aged 27 and younger were firm believers in God, the study found.
Older people hold higher belief in God and it increases with age, may be due to their growing realisation that death is coming closer, the researchers said.
"Looking at differences among age groups, the largest increases in belief in God most often occur among those 58 years of age and older," study author Tom Smith said.
"This suggests that belief in God is especially likely to increase among the oldest groups, perhaps in response to the increasing anticipation of mortality," Smith was quoted as saying by LiveScience.
The study also found that atheism ranged from 52 per cent in the former East Germany to less than one per cent in the Philippines, where about 84 per cent of residents say they have strong belief in God.
The lowest "strong belief" in God came from Japan, where the level was four per cent.
Overall, countries dominated by Catholic societies, like the Philippines, showed the strongest belief in God. Among developed countries, the US stood out for its high belief in God. At the other end of the religion, atheism seemed to have the strongest hold in northwest European countries such as Scandinavia and those of the former Eastern Bloc.
Over the study period, just five of the countries showed a consistent growth in their belief in God: West Germany, Israel, Japan, Russia and Slovenia.
Meanwhile, 16 countries showed a consistent fall in their belief: Australia, Austria, Ireland, UK, the Netherlands, New Zealand, Northern Ireland, Norway, East Germany and Poland. PTI SKP AKJ
