When a protestant raises the question of necromancy I usually respond with the following:
We first look at Deu 18:10-11 where the protestant finds the condemnation of the practice. After reviewing the verses it becomes clear that scripture is talking about the occult and the practices of the occult. The catholic church has always and still does condemn such practices, and they are totally unrelated to prayer to the saints.
I next show them the description of the Transfiguration in Mat 17:2-3 where we see that Jesus is talking with Moses and Elijah and that the apostles are aware of everything that’s going on. Is Jesus disobeying and practicing necromancy? Not a chance!
I also read to them Mat 22:32 where Jesus asks if his listeners had not read where God said, “I am the God of Abraham, the God of Isaac, and the God of Jacob?” Then Jesus immediately says, “He is God not of the dead but of the living.” The saints are not dead, but are instead more alive than we are. Those that are in the abode of the dead [not heaven] are not to be conjured up by soothsayers…that is necromancy.
I also point out Heb 12:1 that says, “Therefore since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight and the sin that clings so closely…” Obviously, this cloud of witnesses are those that have preceded us to heaven.
Finally, I point to Rev 5:8 which reads, “When he had taken the scroll the four living creatures and the twenty-four elders fell before the Lamb, each holding a harp and golden bowls full of incense, which are the prayers of the saints…” Notice that the twenty four elders carry our prayers to God.
Surely, there can be no argument about prayering to saints because the Book of Hebrews describes them as the “souls of just men made perfect” and the Book of James tells us that “the prayers of a just man availeth much.” Since we are called to make supplications and intercessions of all kinds as pointed out by Paul in his first letter to Timothy, it just makes good sense to seek the help of “the most just” to do it on our behalf. There simply isn’t anything in scripture that would suggest otherwise.