When it is forbidden in the Bible?
I think it’s extremely important to realize that when Catholics say “pray to” that we are not implying that we are ruling out God from the equation all together and relying upon a saint to answer our prayers. We use “pray to” in its archaic meaning of “to request” or “to ask.” So when you hear “prayer to the saints” or “prayer to Mary” it means we are asking or requesting them to pray to God on our behalf. It’s confusing if you do not understand Catholic doctrine. No Catholic prays directly to a saint INSTEAD OF to our Lord, but rather we are invoking them to lift our prayers up to God as they are closer to Him in heaven than we are on earth and they have been purified and perfected by God. Saints are a continuation of the Church on earth and cannot be separated by death.
The practice of intercessory prayer is biblical. Revelations 5:8 makes it clear that those in heaven can hear us and do pray for us.
Read the tracts here on Catholic Answers for a very good explanation:
Christ is a unique mediator between man and God because he is the only person who is both God and man. He is the only bridge between the two, the only God-man. But that role as mediator is not compromised in the least by the fact that others intercede for us. Furthermore, Christ is a unique mediator between God and man because he is the Mediator of the New Covenant (Heb. 9:15, 12:24), just as Moses was the mediator (Greek mesitas) of the Old Covenant (Gal. 3:19–20).
The intercession of fellow Christians—which is what the saints in heaven are—also clearly does not interfere with Christ’s unique mediatorship because in the four verses immediately preceding 1 Timothy 2:5, Paul says that Christians should interceed: “First of all, then, I urge that supplications, prayers, intercessions, and thanksgivings be made for all men, for kings and all who are in high positions, that we may lead a quiet and peaceable life, godly and respectful in every way. This is good, and pleasing to God our Savior, who desires all men to be saved and to come to the knowledge of the truth” (1 Tim. 2:1–4). Clearly, then, intercessory prayers offered by Christians on behalf of others is something “good and pleasing to God,” not something infringing on Christ’s role as mediator.
Necromancy is not a Catholic practice and is forbidden. We are not practicing necromancy because the saints are not dead, but more alive than we are.