According to Schaberg's Resurrection of Mary Magdalene, Pope Gregory the Great (540–604) gave a famous homily, praising Mary of Migdal for her penitence, referencing Luke 8 on her previous affliction by seven demons, identifying her as the woman who anointed the feet of Jesus in Luke 7, and also identifying her as the adulteress in John 7–8. Gregory actually suggested that the oil or ointment she used on Christ's feet was what she had used in her earlier life of sin for scenting her body to attract and please her clients. He also equated the seven demons Jesus had cast out of her with the seven cardinal sins, emphasizing that lust was one of them.
Gregory and the others who identified Mary Magdalene as a prostitute may have intended to downplay her importance to Jesus and His early followers.
Curiously, Luke's is the only canonical Gospel that names only Peter as a witness to the resurrected Jesus. Mary Magdalene is named in all the others as a material supporter of Christ's mission and as a witness to the crucifixion, the burial, and the empty tomb. According to Matthew and John, she witnessed the risen Lord as well.
Perhaps the author of Luke's Gospel also sought to downplay her importance, at least in comparison with the other Evangelists. The various Orthodox churches have never done that, but have always held her in high esteem.
Still, it doesn't seem quite fair to dismiss the prostitution claim as mere ignorance of the Bible. It seems to come straight from a papal proclamation. And Gregory the Great was no ordinary pope. He was the one who led a team of scholars in deducing the year of the crucifixion (AD 33) from a careful study of Biblical and historical evidence.
Thank you :)