Can a Catholic in a state of grace be possessed, or does needing deliverance prove he was never saved?

1 John says the one born of God is kept safe and the wicked one does not touch him, but that is a general statement. You have to read it alongside 1 John 1:8-9, which says if we claim to be without sin we deceive ourselves, and 1 John 3:9, which says no one born of God will sin, because God's seed remains in him. Those look contradictory, but 3:9 uses a present-tense Greek verb, so it means he will not continue to sin; the NIV even adds the word "continue." So it does not mean a Christian never sins, only that he does not go on sinning, and the contradiction disappears. Apply the same thing to demons. You may get a little cocky about your faith and start dabbling in Ouija boards or tarot cards, and before you know it things are not normal and the devil or one of his demons is involved; ask Father Ripperger, a good friend of mine who has done many exorcisms. So it is a real possibility, just as it is a possibility that you could fall into mortal sin, the way David murdered his lover's husband. But you have the promise of God's grace to deal with it once you discover you have fallen; you will suffer the temporal punishment, as David's life was never the same, which is why the lesson is: do not dabble in it at all.