The Rt Rev Mark Davies was warning that the country stands at a 'crossroads' over the issue and other matters such as abortion and euthanasia in his Easter homily at Shrewsbury Cathedral tomorrow.
He says Britain risks "falling into darkness" with people unable to distinguish between good and evil due to the marginalisation of Christianity. He has been one of the most outspoken critics of the Government's plans to legalise gay marriage.
He was criticised last year by gay rights group Stonewall, who said his stance went against 'the core message of Christianity being love and respect for all'. But the bishop has maintained his opposition and now says that the country needs to decide whether its laws and culture will be shaped by the Christian faith of its history in the future.
The bishop says: "Commentators have been puzzled that the Church's concern for the poorest is combined with an uncompromising defence of marriage as the union of man and woman; of the family as the vital unit of society; of the unborn routinely destroyed, frozen, manipulated for our purposes; and of the dignity of the aged threatened in many societies by a killing which calls itself 'mercy'.
"It makes the question of what a human life is worth the most urgent question of our time.
"We can see this in the misuse of science or in legislators seeking to redefine the value of human life or to remake marriage.
"If the Christian roots of our society are finally severed what will be left to uphold human dignity, to protect human rights and the value of human life itself?
"Our country stands at the crossroads in deciding on what the future of our life and laws will be based."
Yesterday the former archbishop of Canterbury Lord Carey, accused David Cameron of making Christians feel marginalised.
He said it was a "bit rich" for the prime minister to tell religious leaders to face down aggresive secularisation when the government seemed to be "aiding and abetting" it.
"Many Christians doubt his sincerity," he added and pointed out that in a recent survey more than two-thirds of Christians felt that they were part of a 'persecuted minority'.