Immaculate conception

Immaculate Conception is the Roman Catholic version of the Virgin[sic] Mary's conception without original sin. It is nothing to do with the impregnation of said female by the "Holy Ghost".

The Immaculate Conception is considered a key tenet of Roman Catholicism, and explains why no element of Jesus' nature was stained by original sin. However, despite having a long tradition, it was frequently debated and argued over within the church until 1854 when pope Pius IX decided to put it down in writing (and it wasn't until Pius XII's encyclical of 1943 that Mary was declared to be free of all sin). The notion of Immaculate Conception was stated ex cathedra and is accordingly held to be unalterable and infallible; it is one of only two infallible Papal revelations for Catholics. To deny the Immaculate Conception is thus heretical.

Duns Scotus famously argued for the Immaculate Conception on the grounds of potuit, decuit, ergo fecit — God could have done, it would have been good for God to do so, so God must have done it.