In his Introduction to Christianity, Professor Joseph Ratzinger, now Pope Benedict XVI, gave warning against the “secularisation of love” as preached by Luther, the founder of the Protestant Reformation.

This text is almost certainly among the documents that Tony Blair will have been given to read as part of his “instruction” before becoming a Catholic.

The former Prime Minister will have received an intense programme of spiritual and doctrinal instruction from Father Mark O’Toole, secretary to the Archbishop of Westminster, Cardinal Cormac Murphy-O’Connor. For doctrinal instruction, he will also have been asked to read the Church’s Catechism, which contains all contemporary doctrine as well as other introductions to Christianity.

Chris Gillibrand, a former Anglican who now blogs on Catholic Church conservation from Brussels, said: “However formal or informal the instruction, the Catechism frames the spiritual life of every convert. The existence of the Catechism means there is not a doctrinal continuum between the Catholic Church and other churches, not least the Anglican.”

Speaking of his own conversion, Mr Gillibrand said: “I said goodbye to the Church of England on the steps of Westminster Abbey, went for several beers and a curry in Soho with a good agnostic friend, returning to Oxford that evening by coach which passed by Littlemore, where John Henry, later Cardinal, Newman was received into the Church.

“I was received on the Sunday morning in a frankly ugly church, but I knew, with the hands of the two sponsors on my shoulders, I would be for the rest of my life home.”