On Tuesday, MPs will make a momentous decision on gay marriage. The Catholic Church and Church of England, amongst other religious groups, have opposed the change. But their religious freedom will be protected.

No church or mosque or synagogue will be forced to conduct a gay marriage against their will. As three of the country's leading QCs said in a letter to The Times on Monday, it's "simply inconceivable" that the courts would overturn safeguards in the legislation. Nor will any teacher be forced to "promote" a gay marriage. And as Rabbi Julia Neuberger has argued, religious freedom cuts both ways. Why should the law prevent liberal Jews, Quakers or Unitarian churches from conducting gay marriages, as they wish to do?

For some, the objection is to homosexual conduct itself. Today, that is a minority view, and one thankfully in decline. But I believe that many who do not share this view nevertheless have a principled concern that gay marriage would mean re-defining the institution for everyone. Yet Parliament has repeatedly done this. If marriage hadn't been redefined in 1836, there wouldn't be any civil marriages. If it hadn't been redefined in 1949, under 16-year-olds would still be able to get married. If it hadn't been re-defined in 1969, we wouldn't have today's divorce laws. And all of these changes were opposed.

Much of the argument has been about the electoral impact on the Conservative Party. But all independently conducted polls show that a substantial majority of the public are in favour, and this number is rising fast. We must take care not to mistake vociferous pressure from a few for the quieter view of the many.

But I hope MPs will look beyond their postbags and inboxes and reflect on the real issue at stake. I think of the gay children still bullied at schools, or fearful about whether friends and families will accept them. I think of sportsmen and women, role models who still don't feel able to come out. I know the signal that Parliament will send about whether the law fully recognises the place of gay people in our society will really matter.

Above all, I think of two people, faithful and loving, who simply want their commitment to be recognised as it is for straight couples. We can't just keep telling them that civil partnerships will do. People choose marriage for a reason: they know that it means something special.

This is far bigger than a debate about party management. It would be terrible if MPs used this vote to register some kind of protest, or thought that it didn't count.

Millions will be watching us: not just gay people, but those who want to live in a society where people are treated equally and accepted for who they are. They will hear our words and remember our votes. I hope that MPs will search their consciences, and do the right thing.

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