Many people wonder why we as Catholics call upon Mary and the Saints in intercessory prayer. It is because we firmly believe, and rightfully, that to call upon those truly dead in both body and spirit constitutes necromancy. But for those whose spirit lives on, are they truly dead? Jesus tells us “Whosoever believeth in me shall have eternal LIFE!” So it is established that in doing our best to dedicate our lives to Him, we shall be eternally alive when our mortal shells expire. And thus anybody in Heaven (and Purgatory since Purgatory can only lead to Heaven) can pray for us to the Lord our God. thus we are not practicing necromancy. Also in the case of Mary, since she is conceived without sin, her body would not expire as ours does, since it is because of original sin that our bodies expire the way they do. So that is why in her case we also believe she was taken by God up to heaven (i.e., assumed, not ascended as Jesus did, into Heaven), body and soul.
So we do have the authority to call upon those in Heaven or on their way to Heaven through Purgatory to pray for us. For it is established that such people are eternally alive in Christ. We can also ask our dead ancestors “If you are in Heaven, pray for us.” That is because in their case we aren’t as certain of their eternal fate. However when calling upon Mary and the Saints to pray for us we are truly certain that they are in Heaven because of the lives they led in the natural world of complete dedication to Our Lord. So we can call upon them in intercessory prayer just as well as we can our fellow man.
That is our take on intercessory prayer in a nutshell.