Read the article I posted above, it gives a clear understanding of this.
There is:
1)Actual mortal sin where a person commits the sin with full knowledge, full consent, and grave matter. Actual Mortal sin sends one to hell.
2)Objective mortal sin where there is not full knowledge, so objective mortal sin, although grave does much harm to the body of Christ but will not send one to hell but could send one to Purgatory for a long time.
A good example of this is a bad priest who gives advice to a parishioner that contraception is allowed in some circumstances (although we know contraption is intrinsically evil and allowed in no circumstances) and the next day the parishioner, sort of hesitant, uses contraption to the advice of the priest. It’s grave matter but the person was misled by bad advice, so it wasn’t an actual mortal sin and the person is still in the state of grace. Shame on the priest though for leading a sheep astray. The sin will be on him.