The proportion of British Christian population would reduce by almost a third by 2050 to stand at just 45.4% compared to 64.3% in 2010.
The number of Christians fell by 4.1 million over 10 years in the UK. This decline is fastest among youngsters - 32% of those under 25.
Church of England`s recent data has showed it was attracting fewer than 800,000 people to churches to Sunday masses.
England`s top Christian leader, the Archbishop of Canterbury, recently sparked a debate by admitting to feeling doubts over the existence of God.
The Pew report said Christianity was globally the world`s largest religion, with an estimated 2.2 billion adherents, nearly a third (31%) of all 6.9 billion people in 2010. Islam was second, with 1.6 billion adherents, or 23% of the global population.
Globally, about 40 million people were projected to switch to Christianity, while 106 million were projected to leave it, with most becoming atheists.
The unaffiliated are expected to add 97 million people and lose 36 million due to switching, for a net gain of 61 million by 2050.
Muslims would make up about 10% of Europe`s population, up from 5.9% in 2010, according to Pew projections released on Thursday. Their numbers in Britain are predicted to more than double to 11.3%.
The number of Hindus in Europe is expected to roughly double, from a little under 1.4 million (0.2% of Europe`s population) to nearly 2.7 million.
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