The leaders of the Catholic Church have long opposed abortion.
Under the fifth commandment – "thou shall not kill" – the Catechism of the Catholic Church ruled abortion a "moral evil" and said an embryo must be "treated from conception as a person".
Abortion is punishable under Canon law by excommunication (expulsion from the church and condemnation to hell in the afterlife).
Although firm in the belief that abortion was sinful, Pope Francis last year asked priests to "absolve of the sin of abortion" for women who had "procured it and ... seek forgiveness".
"I think that it is a huge mistake people make, conflating Catholicism with being anti-abortion," Catholics for Choice president Jon O'Brien, an Irishman and lifelong Catholic, told BuzzFeed News.
He has worked on abortion rights in both the United States and Europe, including Poland and Ireland, and played a leadership role in reforming UK abortion laws.
"The leaders are very against abortion but what you find is that Catholics themselves have very different views than the Catholic hierarchy, and use contraception and have abortions at the very same rate as other religions or people with no religion. Ninety-nine per cent of sexually active Catholic women use a form of contraception that the church doesn’t agree with."
At the heart of Catholic teachings, he said, was a "deep regard for following your personal conscience".
"Catholics have a long history of great social justice traditions and our religion has a perspective that allows people to be able to make the best decisions for themselves and their families," O'Brien said, "but the church's hierarchy is considerably invested in making sure people don't have choices."
O'Brien will this week be travelling and speaking around Australia, where a push to decriminalise abortion is on in Queensland and New South Wales.
"Australia is an example of a place which has experienced British colonial rule," ," he said, "like Asia and Africa, and where Britain's Offences Against the Person Act 1861 has formed the basis for abortion to become illegal."