What does it mean by “Selling soul to Devil”?
For some Christians, it would simply mean making a pact with the Devil or simply dealing with the Devil.
A deal with the devil (also called a Faustian bargain or Mephistophelian bargain) is a cultural motif in European folklore, best exemplified by the legend of Faust and the figure of Mephistopheles, as well as being elemental to many Christian traditions. According to traditional Christian belief about witchcraft, the pact is between a person and Satan or a lesser demon. The person offers their soul in exchange for diabolical favours. Those favours vary by the tale, but tend to include youth, knowledge, wealth, fame, or power.
It was also believed that some people made this type of pact just as a sign of recognising the minion as their master, in exchange for nothing. Nevertheless, the bargain is considered a dangerous one, as the price of the Fiend's service is the wagerer's soul. The tale may have a moralising end, with eternal damnation for the foolhardy venturer. Conversely, it may have a comic twist, in which a wily peasant outwits the devil, characteristically on a technical point. The person making the pact sometimes tries to outwit the devil, but loses in the end (e.g., man sells his soul for eternal life because he will never die to pay his end of the bargain. Immune to the death penalty, he commits murder, but is sentenced to life in prison).
The pact can be either oral or written. An oral pact may be made by means of invocations, conjurations, or rituals to attract the demon; once the conjurer thinks the demon is present, he/she asks for the wanted favour and offers his/her soul in exchange, and no evidence is left of the pact. But according to some witch trials, even the oral pact left evidence, the Witches' mark, an indelible mark where the marked person had been touched by the devil to seal the pact. The mark could be used as a proof to determine that the pact was made. It was also believed that on the spot where the mark was left, the marked person could feel no pain. A written pact consists in the same forms of attracting the demon, but includes a written act, usually signed with the conjurer's blood (although sometimes it was also alleged that the whole act had to be written with blood; meanwhile some demonologists defended the idea of using red ink instead of blood and others suggested the use of animal blood instead of human blood).
The Malleus Maleficarum discusses several alleged instances of pacts with the Devil, especially concerning women. It was considered that all witches and warlocks had made a pact with one of the demons, usually Satan. - Deal with the Devil
Legend has it that St. Theophilus of Adana made a pact with the Devil and later repented of his dealings with Satan.
Saint Theophilus the Penitent or Theophilus of Adana (died c. 538 AD) was a cleric in the sixth century Church who is said to have made a deal with the Devil to gain an ecclesiastical position. His story is significant as it is the oldest story of a pact with the devil and was an inspiration for the Faust legend. Eutychianus of Adana, who claimed to be an eyewitness of the events, is the first to record Theophilus's story.
Although Theophilus is considered to be an historical personage, the tale associated with him is of an apocryphal nature. His feast day is February 4.
To be a Satanist implies that one has dealings with the Devil and that one has made a pact with the Devil, thus selling one’s soul to Satan for eternity in hell.
However, we do hear of some breaking their diabolical pacts with the enemy of mankind, truly repenting of their evil doing and becoming model Christians.
One such case is Bartolo Longo, who was a former Satanic priest.
Bartolo Longo (February 10, 1841 – October 5, 1926) was an Italian lawyer who has been beatified by the Roman Catholic Church. He was a former satanic priest who returned to the Catholic faith and became a third order Dominican, dedicating his life to the Rosary and the Virgin Mary. He was eventually awarded a papal knighthood of the Order of the Holy Sepulchre.
In the 1860s, the Catholic Church in Italy found itself at odds with a strong nationalistic movement. General Giuseppe Garibaldi, who played a key role in Italian unification, saw the Pope as an antagonist to Italian nationalism and actively campaigned for the elimination of the papal office altogether.3 The Catholic Church in Europe was also competing with a growing popularity in Spiritualism and Occultism. Because of this, many students at the University of Naples took part in demonstrations against the pope, dabbled in witchcraft and consulted Neapolitan mediums. Longo became involved with a movement that he claimed led him into a Satanist cult. After some study and several "spiritual" experiences Longo said that he was ordained as a satanic priest.
In the following years, Longo's life became one of "depression, nervousness, and confusion". Bothered by paranoia and anxiety, he turned to a hometown friend, Vincenzo Pepe, for guidance. It was Pepe who convinced him, in Longo's account, to abandon Satanism and introduced him to the Dominican Father Alberto Radente who led him to a devotion to the rosary. On October 7, 1871, Longo became a Dominican tertiary and took the name "Rosario". Around this time, he reportedly visited a séance and held up a rosary, declaring, "I renounce spiritualism because it is nothing but a maze of error and falsehood." He also came to know some Franciscans with whom he helped the poor and incurably ill for two years. Bartolo also kept up his law practice, which took him to the nearby village of Pompei. He went to Pompei to take care of the affairs of Countess Marianna Farnararo De Fusco.
In Pompei, Longo later recounted, he was shocked at the erosion of the people's faith. He wrote, "Their religion was a mixture of superstition and popular tradition. ... For their every need, ... they would go to a witch, a sorceress, in order to obtain charms and witchcraft." Through talking to the citizens, Bartolo came to recognize their severe lack of catechesis. When he asked one man if there was only one God, the fellow answered, "When I was a child, I remember people telling me there were three. Now, after so many years, I don't know if one of them is dead or one has married."
Longo wrote of his personal struggles with mental illness, paranoia, depression and anxiety. At one point, he noted struggling with suicidal thoughts, but rejected them by recalling the promise of Saint Dominic, "he who propagates my Rosary will be saved." Longo wrote that this promise is what convinced him to encourage public devotion to the rosary.
Blessed Gil of Santarem is a Dominican Blessed who repented of making a pact with the Devil.
Blessed Gil of Santarem
A Portuguese Dominican: b. at Vaozela, diocese of Viseu, about 1185; d. at Santarem, 14 May, 1265. His father, Rodrigo Pelayo Valladaris, was governor of Coimbra and councillor of Sancho I. It was the wish of his parents that Gil should enter the ecclesiastical state, and the king was very lavish in best caving ecclesiastical benefices upon him. When he was still a boy, he already held prebends at Braga, Coimbra, Idanha, and Santarem. Gil, however, held no desire to be an ecclesiastic; his ambition was to become a famous physician. After devoting some time to the study of philosophy and medicine at Coimbra he set out for Paris, with the intention of perfecting himself in the science of medicine and obtaining the doctor's degree. If we may give credence to his unknown contemporaneous biographer, he was accosted on his journey by a courteous stranger who promised to teach the art of magic at Toledo. As payment, so the legend runs, the stranger required that Gil should make over his soul to the devil and sign the compact with his blood. Gil obeyed and after devoting himself seven years to the study of magic under the direction of Satan, went to Paris, easily obtained the degree of doctor of medicine, and performed many wonderful cures. One night while he was locked up in his library a gigantic knight, armed head to foot, appeared to him and, with his sword drawn, demanded that Gil should change his wicked life. The same spectre appeared a second time, and threatened to kill Gil if he would not reform. Gil now repented of his evil ways, burnt his books of magic and returned to Portugal, where he took the habit of St. Dominic in the newly-erected monastery at Palencia, about 1221. Shortly after, his superiors sent him to the Dominican house at Scallabis, the present Santarem. Here he led a life of prayer and penance, and for seven years his mind was tormented by the thought of the compact which was still in the hands of Satan. Finally, his biographer narrates, the devil was compelled to surrender the compact and place it before the altar of the Blessed Virgin. Gil returned to Paris to study theology and on his return to Portugal became famous for his piety and learning. He was twice elected provincial of his order in Spain. Benedict XIV ratified his cult on 9 March, 1748.