Today, December 8th, is the Feast of the Immaculate Conception. Know why? Nine out of 10 people get it wrong. They think it refers to the conception of Jesus. It refers to the conception of his mother, Mary.

You would have thought the date a giveaway. Or do people forget that the Feast of the Annunciation on March 25th marks the conception of Jesus, nine months before Christmas Day? Mary’s birthday is September 8th.

It is such an odd word, immaculate. From the Latin immaculatus, meaning unstained, pure, free from mental or moral pollution.

According to Catholic Church teaching Mary was free of original sin from the moment of her conception by her mother. Who was . . . .? Stop prompting at the back of the class. You're wrong, as usual.

Mary’s mother was St Anne and her father . . . was St Joachim.

You didn’t know that either. Stop pretending.

Anyhow, it would be argued that belief in the immaculate conception of Mary the mother of Jesus has been part of Catholic tradition since time immemorial. Not so with Protestants however, for whom Mary is somewhat of a lesser personage in the theological scheme of things. But it is contended that the immaculate conception of Mary is also a teaching of Islam. An entire chapter of the Koran is devoted to her.

What will surprise many Catholics is that the Immaculate Conception only became a dogma of the church in 1854, when it was proclaimed as such by Pope Pius IX, or Pio Nono as he was known – not always affectionately – to contemporaries.

In his Ineffabilis Deus, proclaimed 160 years ago today on December 8th, 1854, he said "We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful."

It was that great Irish medieval theologian Blessed John Duns Scotus (died in 1308) who argued that God sanctified Mary at the moment of conception with the foreknowledge that she would consent to bear Jesus. Thereafter, celebrations of the feast spread through western Christendom.