Leader of the Catholic church in Scotland says Christianity is being marginalised as PM invites churchmen to Downing Street

The leader of the Catholic church in Scotland has used his Easter address to attack "aggressive secularism", suggesting there were "those who would indeed try to destroy our Christian heritage and culture and take God from the public square".

He made the address as David Cameron publicly endorsed the "enormous contribution" of Christian values to Britain, days before he welcomes senior churchmen to Downing Street for an Easter celebration.

Cardinal Keith O'Brien, archbishop of St Andrews and Edinburgh, who is known for his outspoken views, eschewed traditional Easter themes of resurrection and hope and instead set his sights firmly on the "marginalisation" of Christianity in the UK.

He told the congregation at St Mary's Cathedral; "Christians must be united in their common awareness of the enemies of the Christian faith in our country, of the power that they are at present exerting, and the need for us to be aware of that right to equality which so many others cry out for."

His homily included instances where Christians had fallen foul of equality legislation, preventing discrimination against gay people, and swiftly drew fire from groups campaigning against religious privilege in public life.

Keith Porteous-Wood, from the National Secular Society, said religious leaders were becoming "increasingly paranoid by the mass exodus from their churches" and were blaming the "bogeyman of secularism" rather than their own "narrowmindedness and bigotry".

In his Easter message the prime minister said: "Jesus taught us to love God and love our neighbour. He led by example and for millions of us his teachings are as relevant now as they were in his lifetime. As we share in this festival with our friends and family, we can all be reminded of the enormous contribution Christianity has made to our country."

Wednesday's No 10 meeting – which embraces various denominations – will be seen as an endorsement of Britain's Christian heritage and identity amid a perceived climate of anti-religious sentiment.

It is the most explicit outreach to Christian groups in Britain by a government in recent times. Prior state-led engagement with religion has focused almost exclusively on Muslims and the Islamic faith.

Baroness Warsi is one of several cabinet figures wooing church leaders. Earlier this month she was the keynote speaker at a Roman Catholic conference addressing the issue of social responsibility, while last year she unequivocally told Anglican bishops that, unlike the previous administration, the coalition did "do God".

Andrew Copson, from the British Humanist Association, described O'Brien's remarks as "ill-informed" and "alarmist" and accused him of sectarianism.

He said: "What these attacks ignore is that campaigners for secularism in our public life are overwhelmingly motivated, not by anti-religious prejudice, but by a positive desire for equality and an equitable public sphere.

"These alarmist speeches, designed to stir up the faithful and foster a false narrative of persecution, are divisive and sectarian."

Such attacks "obscured" the reality of the situation, he said. "The churches are seeking to defend a level of influence and privilege totally out of proportion to their significance," Copson added.

The plight of British Christians – and the sidelining of their faith – also figured in the Easter Sunday homily of the Roman Catholic archbishop of Westminster.

The Most Rev Vincent Nichols stepped into the debate on assisted suicide. He told his congregation that the "light of faith" gave meaning to their lives and that those without it risked ending their lives – or even the lives of others.

"Without this faith so much is missing from life. Without such faith we can become afraid of living. Indeed, in pain and loneliness, or in even the prospect of pain and loneliness, life, for some, loses its purpose and killing oneself or a loved one becomes a beguiling temptation.

"Those who belittle faith and ridicule the things we cherish do so from a distance, with little understanding of the true reality of a relationship with the Lord. We know better because we live these truths of faith."

A spokesman for Dignity in Dying Dignity in Dying said "a clear majority" of religious people supported a change in the law on assisted dying – 71% according to the 2010 British Attitudes Survey.