The Catholic practice of prayer intercession, also known as the intercession of saints, dates back to the New Testament and the early writings of the church in 329. In the New Testament, Saint Paul writes to fellow Christians in Ephesus, Colossus and Rome to pray on his behalf. In the early writings of the church, Saint Basil invokes the saints, prophets and apostles to intercede on his behalf.
Early Catholics believed that martyrs would go straight to Heaven after death and could also obtain blessings and prayers for others still on Earth. This led to the practice of prayers of intercession to the saints on behalf of the petitioners below. This is a common practice in Catholic, Eastern Orthodox and some Anglican orders of Christianity.
Modern Protestant believers disagree with this practice, citing the doctrine that Jesus Christ is the only mediator between humans and God. Modern Protestants believe that it is acceptable to ask other Christians to pray with you or for you, but they do not believe in the intercession of saints who have died. Catholics disagree, arguing that while Christ may be the sole mediator, there is no reason to believe the saints cannot intervene on their behalf as well. In addition to the invocation of saints, Catholic practitioners also call on the Virgin Mary in their Hail Mary prayers.