The Archbishop of Canterbury has told a group of Christians that people will view their stance on gay marriage as "wrong and wicked" and urged them to repent.
Speaking to a group of born-again Christians at opening of the Evangelical Alliance's new premises in King's Cross, London, the Most Rev Justin Welby said the Church must now change its "utterly and totally wrong" views on gay marriage to fit in with society.
Welby, who voted against same-sex marriage in the House of Lords and once opposed allowing gay couples to adopt, said that opposing the move would be seen by a majority of under-35s as parallel to "racism and other forms of gross and atrocious injustice".
He previously said he has had to reassess his own views and has now urged others to face up to a "revolution" in attitudes on sexuality following the landmark ruling on gay marriages.
He said: "What I voted against was what seemed to me to be the rewriting the nature of marriage in a way that I have to say within the Christian tradition and within scripture and within our understanding is not the right way to deal with the very important issues that were attempted to be dealt with in that Bill.
"The Bill was clearly, quite rightly, trying to deal with issues of homophobia in our society.
"As I said at the time in the House of Lords, the Church has not been good at dealing with homophobia - it has at times, as God's people, either implicitly or explicitly supported it and we have to be really, really repentant about that because it is utterly and totally wrong."
"If the same thing happened again I would vote the same way as I did then but I am continuing to think and listen very carefully as to how in our society today we respond to what is the most rapid cultural change in this area that there has been, well, I don't know if ever, but for a very long time.
"And we have seen changes in the idea about sexuality, sexual behaviour. We have to face the fact that the vast majority of people under 35 think not only that what we think is incomprehensible but also think that we are plain wrong and wicked and equate it to racism and other forms of gross and atrocious injustice."
Welby also suggested that that the majority of Christian young people would also disagree with the Church's traditional stance on homosexuality.
"We have to be real about that, I haven't got the answer and I'm not going to jump one way or the other until my mind is clear about this," he added.
Benjamin Cohen, founder of campaign group Out4Marriage and publisher of PinkNews, added: "It is welcome that the Archbishop of Canterbury has recognised that the majority of people under the age of 35 do not consider same-sex relationships as anything other than normal.
"They do see that attacking gay people for the gender of the person that they love is as evil and incomprehensible as attacking someone for being born black or disabled. People don't chose to be gay just like they don't chose their race.
"I would not argue that the Archbishop's stance was 'wicked'. All the way through the debate on same-sex marriage, those of us in favour of the change always maintained that churches, synagogues and mosques should be free not to opt-in to same-sex marriage. This is their right. Just as it is the right of the younger generations to question the relevance of these institutions if they reject a change in the law that most young people think is nothing more than equality."
Ben Summerskill, chief executive of gay rights charity Stonewall, added: "It is a tiny bit rich to say he has great sympathy for gay people when in the 10 years since the introduction of civil partnerships the Church has doggedly refused to bless people's long term partnerships even though they are happy to have services for pets and even canals."