Jim Wallis, author, social activist and faith adviser to the Prime Minister, has backed the conclusions of a new Theos report published on Saturday arguing that the church must think carefully before partnering with government.

The report by Theos, the public theology think tank, called Neither Private nor Privileged, examines the role of Christianity in Britain today. It rejects calls to privatise religious faith but insists that the nature of the church's involvement with government should differ according to the 'moral orientations' of the state at any given time.

The role of Christianity should, therefore, be adjudicated on a case-by-case basis. Underpinning this approach, the contribution of Christianity in the public square will depend on the extent to which, by doing what it must do, it can persuade the public that it is 'doing good'.

In his foreword to the report, Jim Wallis writes: "The church... has the obligation to closely examine the moral orientation of the state, how its policies and actions contribute to the public good, not religious belief; and measure them against the church's conception of the public good that underlies its public witness. This provides the framework to determine the appropriate response.

"We should reject allegedly utopian or perfect societies, which are impossible creations for sinful people in a fallen world. We should rather seek concrete reforms of the social situations and circumstance in which the church finds itself... And, as the report correctly points out, for Christians [this means] that we seek the 'common good' of the societies in which we live."

In 2006 Gordon Brown personally endorsed a book by Wallis on faith in politics which was critical of the 'unjust' Iraq war. It portrayed the then Chancellor as a spiritual thinker after the American pastor developed an unexpected friendship with him. Wallis is also an advisor to Barack Obama and others in the Democrat party on faith issues. He is due to visit the UK in June to promote his new book Seven Ways to Change the World.

Commenting on the Theos report, the Rt Revd Michael Nazir-Ali, Bishop of Rochester, said: "The rejection of both theocracy and the privatisation of faith will ring true for many, as will its affirmation of the duty and value of Christian presence, proclamation and praxis."

Lord Harries of Pentregarth, the former Bishop of Oxford, added: "The question of the role of religion in a modern society is one of the most relevant and interesting issues facing us. I am delighted to welcome this contribution of Theos to this debate."