Local churches around the UK have a spiritual and moral responsibility to support the new Coalition for Marriage launched this week.

This campaign is a theological litmus test for the UK Church. Churches that refuse to support C4M demonstrate that they have abandoned the authority of the Bible.

C4M, which is backed by the Christian Institute, CARE, Christian Concern, the Evangelical Alliance, and the Family Education Trust, is gathering signatures for a petition against Mr Cameron's bid to redefine marriage.

If the leadership of the Church of England fails to get behind C4M, then that will be an indication that corrosive theological liberalism, so destructive of mission, has gained control of the institutional hierarchy.

Under that scenario, Anglican evangelicals would need to be pro-active both in supporting C4M and in denouncing their denominational leadership for failing to uphold the truth of God's Word.

Unfortunately, a fudge on marriage by the Church of England bishops would be indicative of the failure of the Anglican evangelical strategy since the 1970s. Back in the days of T-Rex and flared trousers, Anglican evangelical leaders started to encourage younger ministers to engage with the denominational structures of the Church of England. This led in the 1980s and 1990s to a greater number of evangelicals being appointed archdeacons and bishops. The idea was to transform the Church of England by the institutional route.

Some institutional transformation 35 years on if the House of Bishops cannot unite to stand up against same-sex marriage. The fragility of the bishops' collegiality behind the sanctity of heterosexual marriage was exposed when the Bishop of Salisbury Nicholas Holtam broke ranks earlier this month with supportive noises in The Times in favour of homosexual 'marriage'.

Lord Carey, appointed Archbishop of Canterbury in 1990, was a product of the post-war Anglican evangelical inside strategy. He is the most prominent public figure behind C4M. It is significant to note how much more bold and clear he has become in speaking up for biblical truth since he stopped being Archbishop in 2002.

He never abandoned the biblical convictions he learned from the Dagenham vicar who led him to the Lord Jesus Christ as a teenager in the early 1950s. But freed from the straightjacket of leadership in a theologically eclectic denomination he now feels free to express them. The rapid advance of political correctness in British national life in the past decade has roused him to speak out from his platform in the House of Lords.

George Carey's support for C4M is the culmination of a string of campaigns he has waged against politically correct legislation not only on issues affecting marriage and the family but on freedom of Christian expression.

Leaving office seems to have brought out the attacking footballer in Dr Carey. At times, he has been the George Best of the fight against anti-Christianity in the UK - creative, resourceful and stylish.

The C4M petition to Her Majesty's Government is very well worded: "I support the legal definition of marriage which is the voluntary union for life of one man and one woman to the exclusion of all others. I oppose any attempt to redefine it."

But the petition is not enough. A rally featuring Bible teaching, Christian advocacy and public prayer against the destructive politically correct drive to countermand the Word of God is imperative.

A petition, however long, presented to the Prime Minister by a delegation does not have the same impact as a mass demonstration in central London.

C4M should honour the historic legacy of the great public campaigns of Britain's Christian past by spearheading this peaceful mass demonstration for marriage. Past Christian campaigns such as those against the slave trade in the 18th century and against industrial exploitation in the 19th were advanced by large-scale public meetings.

The phenomenon of an orderly, respectful rally for a good cause grew in the biblically-enriched cultural soil of a Christian-influenced Parliamentary democracy.

If local churches around the country cannot get their members out in significant numbers for a rally on an issue as biblically central as this one, then public Christianity in the UK would be sounding its own death-knell by default.

The consequences of same-sex marriage for local churches, Christian outreach networks to children and young people, and church schools would be catastrophic. Political correctness would be given free rein to corrupt the minds of the next generation with its dangerous ideology.

The Lord Jesus Christ publicly defended the God-created institution of monogamous, heterosexual, life-long marriage. He did so before large crowds in the course of his public ministry in Galilee and argued the biblical case against powerful vested interests (see Matthew 19v1-12 and Mark 10v1-12).

We must take his lead and avail ourselves of the privilege of peaceful public demonstration, which faithful British Christians in the past campaigned for at great personal cost and exercised.