The chief executive of the Lesbian and Gay Christian Movement is "delighted that love and justice have won the day" in the latest vote on equal marriage.
But she also warned that there might be further attempts to thwart change, and that prayerful vigilance was necessary.
The Rev Sharon Ferguson was commenting in the immediate aftermath of the House of Lords voting 390 to 148 in favour of the Marriage (Same Sex Couples) Bill at its second reading.
This measure allows what its proponents have dubbed 'equal marriage' in England and Wales. Separate but similar proposals are being promulgated in Scotland.
At Westminster, the measure was backed by a sizeable majority in the House of Commons, and will now go to Committee stage in the Lords.
While many churches and Christian organisations have campaigned against the Bill, others have supported it, regarding it as good for society and fully in keeping with their religious values.
A sizeable number of Christians now believe that equal marriage should be welcomed by society and celebrated in churches, though the Bill upholds religious freedom and faith groups can choose whether or not to opt into celebrating relationships in their buildings.
"I would like to thank all our members, supporters and allies who have put so much effort into working for marriage equality," declared Ms Ferguson.
"But we should not be complacent," the LGCM chief executive added. "As the Bill is discussed and amendments put forward, there is a risk that further attempts will be made to wreck it. So please keep writing to peers, speaking out and praying."
Faith bodies and persons backing same-sex marriage provision include the Religious Society of Friends (Quakers) in Britain, the United Reformed Church, the Anglican Bishop of Salisbury, the Suffragan Bishop of Buckingham, Liberal Judaism, the Unitarian and Free Christian Churches, the Metropolitan Community Churches, Accepting Evangelicals, the Lesbian and Gay Christian Movement, Changing Attitude, Christians for Equal Marriage, Inclusive Church, evangelical Baptist figurehead the Rev Steve Chalke, and the Christian think-tank Ekklesia.