Objection 1. It would seem that divination practiced by invoking the demons is not unlawful. Christ did nothing unlawful, according to 1 Peter 2:22, "Who did no sin." Yet our Lord asked the demon: "What is thy name?" and the latter replied: "My name is Legion, for we are many" (Mark 5:9). Therefore it seems lawful to question the demons about the occult.
Objection 2. Further, the souls of the saints do not encourage those who ask unlawfully. Yet Samuel appeared to Saul when the latter inquired of the woman that had a divining spirit, concerning the issue of the coming war (1 Samuel 28:8, sqq.). Therefore the divination that consists in questioning demons is not unlawful.
Objection 3. Further, it seems lawful to seek the truth from one who knows, if it be useful to know it. But it is sometimes useful to know what is hidden from us, and can be known through the demons, as in the discovery of thefts. Therefore divination by questioning demons is not unlawful.
On the contrary, It is written (Deuteronomy 18:10-11): "Neither let there be found among you... anyone that consulteth soothsayers... nor... that consulteth pythonic spirits."
I answer that, All divination by invoking demons is unlawful for two reasons. The first is gathered from the principle of divination, which is a compact made expressly with a demon by the very fact of invoking him. This is altogether unlawful; wherefore it is written against certain persons (Isaiah 28:15): "You have said: We have entered into a league with death, and we have made a covenant with hell." And still more grievous would it be if sacrifice were offered or reverence paid to the demon invoked. The second reason is gathered from the result. For the demon who intends man's perdition endeavors, by his answers, even though he sometimes tells the truth, to accustom men to believe him, and so to lead him on to something prejudicial to the salvation of mankind. Hence Athanasius, commenting on the words of Luke 4:35, "He rebuked him, saying: Hold thy peace," says: "Although the demon confessed the truth, Christ put a stop to his speech, lest together with the truth he should publish his wickedness and accustom us to care little for such things, however much he may seem to speak the truth. For it is wicked, while we have the divine Scriptures, to seek knowledge from the demons."
Reply to Objection 1. According to Bede's commentary on Luke 8:30, "Our Lord inquired, not through ignorance, but in order that the disease, which he tolerated, being made public, the power of the Healer might shine forth more graciously." Now it is one thing to question a demon who comes to us of his own accord (and it is lawful to do so at times for the good of others, especially when he can be compelled, by the power of God, to tell the truth) and another to invoke a demon in order to gain from him knowledge of things hidden from us.
Reply to Objection 2. According to Augustine (Ad Simplic. ii, 3), "there is nothing absurd in believing that the spirit of the just man, being about to smite the king with the divine sentence, was permitted to appear to him, not by the sway of magic art or power, but by some occult dispensation of which neither the witch nor Saul was aware. Or else the spirit of Samuel was not in reality aroused from his rest, but some phantom or mock apparition formed by the machinations of the devil, and styled by Scripture under the name of Samuel, just as the images of things are wont to be called by the names of those things."
Reply to Objection 3. No temporal utility can compare with the harm to spiritual health that results from the research of the unknown by invoking the demon.
Source: Summa Theologica (New Advent)