Both Father Brennan, a Jesuit priest, and Mr Sheridan, a foreign affairs journalist for The Australian newspaper who is also a Catholic, have publicly supported same-sex marriage in recent months.
Both have argued that a Catholic can in good conscience support same-sex marriage for the common good of society while maintaining a personal belief in sacramental marriage as taught by the Church.
However Dr Connor Sweeney, a lecturer at the John Paul II Institute for Marriage and Family in Melbourne, says their statements to this point are incompatible with the faith.
“While they might still seek to advocate a ‘yes’ vote it needs to be clear that this would involve dissenting from Catholic teaching and place one in an irregular relationship with the Church,” Dr Sweeney wrote in an opinion piece for The Catholic Weekly.
“For Catholics such as Brennan and Sheridan it is incompatible with their faith to support the redefinition of civil marriage to include same sex couples.”
It is not possible for Catholics to support the re-definition of marriage to include members of the same gender, Dr Sweeney wrote.
The Church, however, does recognise civil marriages as real unions.
“A validly contracted natural or ‘civil’ marriage between a man and a woman (that is, a marriage taken up outside of the sacramental and canonical regulations of the Church) is also a real marriage on the basis that it belongs to the enduring integrity of this original ‘order of creation’.”