On January 30, 1649, King Charles I was executed; soon afterwards Eikon Basilike, his partly ghost-written apologia, was published. It rapidly became a bestseller, running through some 50 editions in the first year; no doubt that played its part in building public support for the restoration of the monarchy in 1660. The Eikon can be described only as a High Tory document, arguing that a king and a subject have quite different functions. Whatever else may be said of him, Charles I was neither a liberal nor a democrat.

If one looks for the first recorded example of the use of the word “discrimination” in its modern political sense, one finds it in a rather tortuous sentence in Eikon Basilike. “Take heed of abetting any factions, of applying to any public discriminations in matters of religion.” King Charles always did believe in uniformity in matters of religion; that was the policy of William Laud, Archbishop of Canterbury, and Charles’s enthusiastic supporter. Charles was a “no discrimination” king. That was his advice to his successors.

It has been surprising to find Charles’s doctrine rising up again in the present dispute over the Catholic agencies’ refusal to organise adoption for same-sex couples. It is even more surprising that Charles’s doctrine has been adopted by the Left or liberal wing of politics. Alan Johnson, the Education Secretary, seems to be deciding this adoption issue. He has said that he would never agree to discrimination; “no discrimination” has been echoed by the Government front bench in the House of Lords.

It seems to be the policy of the Liberal Democrats. David Cameron has told The Daily Telegraph that “we shouldn’t put up with this discrimination on the basis of race, age or sexual orientation”. If interpreted literally, that would eliminate the age of consent, which involves discrimination on the grounds of age, and would raise the awkward question whether paedophilia constituted a “sexual orientation”.

Almost all law is concerned with discriminating between different cases that receive different treatment. Even the Civil Partnership Act itself is avowedly discriminatory. Same-sex couples gain substantial tax advantages, equal to those of a married couple. Members of the same family are not allowed to enter into civil partnerships with each other; nor are unmarried heterosexual couples. These seem to me to be unfair discriminations, but that is not the point. They undoubtedly are discriminatory and the exclusion of heterosexual couples is undoubtedly a discrimination based on sexual orientation. It is a matter of same sex, yes, but different sexes no.

Mr Cameron is a thoughtful politician, which makes his views on the adoption issue particularly interesting. He said: “It is time to sweep away failed multiculturalism. I don’t think it would be right to allow carve-outs for Muslim groups or Hindu groups or whoever, so that means one law that everyone has to obey. And that’s why I don’t think a block exemption for Catholic adoption agencies would be right.” Mr Cameron understands that he is attacking multiculturalism. He does not seem to understand that multiculturalism is the basis of liberalism.

If liberalism has a core of meaning, it is that different people, different groups, different churches, different religions, have a right to hold different views. Society has the overriding right to protect itself against anarchy and terrorism, but so far as possible society should leave people free to make their own judgments and decide on their own actions. All voluntary agencies could and should have been left to make their own rules for adoptions. The State could decide the rules for state agencies.

Mr Cameron is in company that would regard itself as liberal. He has most of the Liberal Democrats with him, and most of new Labour with him. He sees himself as a liberal Conservative. But his view that there should be no exceptions in law to allow for differences in religious beliefs is neither liberal, nor workable. It is illiberal because liberty depends on pluralism and therefore has to accept multiculturalism. It is unworkable because Britain has no way of imposing our belief systems on Islam.

Other religions may give way, like the ocean; Islam is like a rock. Wise sailors do not steer into the rocks.

Society has no choice but to act against a church or religion that attacks social order, as some Muslim groups do. In the reigns of Queen Elizabeth I or King James I the English Government was justified in acting against Catholic plots, though many Catholics were unjustly punished and were pushed into extremism. British policy in Ireland has never been forgiven.

When Mr Cameron argues that Catholic adoption agencies should be given “time to find a way through the new rules”, he does sound condescending to most Catholics. The Catholic Church has had a doctrine of marriage, as an indissoluble union between two people of different sexes, taking priority over all other relationships, since the time of Christ, endorsed by his specific words.

The Catholic Church believes that doctrine would be prejudiced if Catholic agencies arranged adoption for same-sex couples. After 2000 years, the Church is not going to change its mind because of a vote in the House of Commons. In matters of faith and morals, the Catholic Church sees itself as sovereign, because it is teaching the doctrine of Jesus. The Church may not break British law, but it will certainly not break its own law.

Harold Macmillan is said to have observed that a prime minister should avoid taking on three bodies, the Brigade of Guards, the Roman Catholic Church and the National Union of Mineworkers. Ted Heath was indeed destroyed by the mineworkers, Margaret Thatcher eventually defeated them. A prudent prime minister knows the limits of his own authority. He can pass laws, but he cannot enforce consent. The great religions command strong loyalties. It would be a great mistake if the British Government decided to take them on, and a pity if the Leader of the Opposition supported a policy of compelled uniformity.

Times Recommends

• The ghost of Robin Cook haunts Chilcot’s feast
• The truth shouldn’t need oaths
• Violence: let’s separate the men from the boys