The Rt Rev. Mark Davies delivered the message to more than 1,000 young Catholics and urged them to make a "clear stand" for their faith after recent Government data which suggest Christians living in Britain will soon become a minority.
The outspoken Shrewsbury bishop told young adults at a five-day prayer festival in Norfolk the results of the last census suggested most Britons would not describe themselves as Christians by 2020.
In a speech to Youth 2000, a movement for Catholics aged between 16 and 30 years, he said a new generation needed to find the courage to offer an "inspirational lead" to their contemporaries.
There are still 41 million Christians in Britain but a recent think-tank warned that 4,000 churches could close by 2020 if congregations continue to shrink at current rates.
Bishop Davies said: "With more than three million in Rio last month and with more than a thousand in Walsingham this weekend, we might not really feel like a minority but that is what Christians are about to become in this country of ours.
"By 2020, if the analysis of the recent census is to be believed, most people in this land will no longer identify themselves in any way as Christians.
"It's a situation you already know well as young people of 21st Century Britain.
"A situation which will surely demand of new generations of Catholics a clear stand, an inspirational lead and, as the Gospel reminds us today, a human struggle."
Bishop Davies has been one of the most outspoken critics of the Government's plans to legalise gay marriage and used an Easter address to criticise the proposals.