"We have to know all about Sanctifying Grace, a pearl of great price"
St Peter with the Keys of Heaven C18th Greek Icon
Creative Commons AttributionSt Peter with the Keys of Heaven C18th Greek Icon - Credit: jojojoe

It is believed by the Roman Catholic Church that Sanctifying Grace will make the soul holy, that is, place it in a state of grace.  On dying, these souls will pass into purgatory or go straight to heaven.  If a mortal sin is committed, they lose this state of supernatural grace.