Objection 1. It would seem that divination by drawing lots is not unlawful, because a gloss of Augustine on Psalm 30:16, "My lots are in Thy hands," says: "It is not wrong to cast lots, for it is a means of ascertaining the divine will when a man is in doubt."
Objection 2. There is, seemingly, nothing unlawful in the observances which the Scriptures relate as being practiced by holy men. Now both in the Old and in the New Testament we find holy men practicing the casting of lots. For it is related (Joshua 7:14, sqq.) that Josue, at the Lord's command, pronounced sentence by lot on Achan who had stolen of the anathema. Again Saul, by drawing lots, found that his son Jonathan had eaten honey (1 Samuel 14:58, sqq.): Jonas, when fleeing from the face of the Lord, was discovered and thrown into the sea (Jonah 1:7, sqq.): Zacharias was chosen by lot to offer incense (Luke 1:9): and the apostles by drawing lots elected Matthias to the apostleship (Acts 1:26). Therefore it would seem that divination by lots is not unlawful.
Objection 3. Further, fighting with the fists, or "monomachy," i.e. single combat as it is called, and trial by fire and water, which are called "popular" trials, seem to come under the head of sortilege, because something unknown is sought by their means. Yet these practices seem to be lawful, because David is related to have engaged in single combat with the Philistine (1 Samuel 17:32, sqq.). Therefore it would seem that divination by lot is not unlawful.
On the contrary, It is written in the Decretals (XXVI, qu. v, can. Sortes): "We decree that the casting of lots, by which means you make up your mind in all your undertakings, and which the Fathers have condemned, is nothing but divination and witchcraft. For which reason we wish them to be condemned altogether, and henceforth not to be mentioned among Christians, and we forbid the practice thereof under pain of anathema."
I answer that, As stated above (Article 3), sortilege consists, properly speaking, in doing something, that by observing the result one may come to the knowledge of something unknown. If by casting lots one seeks to know what is to be given to whom, whether it be a possession, an honor, a dignity, a punishment, or some action or other, it is called "sortilege of allotment"; if one seeks to know what ought to be done, it is called "sortilege of consultation"; if one seeks to know what is going to happen, it is called "sortilege of divination." Now the actions of man that are required for sortilege and their results are not subject to the dispositions of the stars. Wherefore if anyone practicing sortilege is so minded as though the human acts requisite for sortilege depended for their result on the dispositions of the stars, his opinion is vain and false, and consequently is not free from the interference of the demons, so that a divination of this kind is superstitious and unlawful.
Apart from this cause, however, the result of sortilegious acts must needs be ascribed to chance, or to some directing spiritual cause. If we ascribe it to chance, and this can only take place in "sortilege of allotment," it does not seem to imply any vice other than vanity, as in the case of persons who, being unable to agree upon the division of something or other, are willing to draw lots for its division, thus leaving to chance what portion each is to receive.
If, on the other hand, the decision by lot be left to a spiritual cause, it is sometimes ascribed to demons. Thus we read (Ezekiel 21:21) that "the king of Babylon stood in the highway, at the head of two ways, seeking divination, shuffling arrows; he inquired of the idols, and consulted entrails": sortilege of this kind is unlawful, and forbidden by the canons.
Sometimes, however, the decision is left to God, according to Proverbs 16:33, "Lots are cast into the lap, but they are disposed of by the Lord": sortilege of this kind is not wrong in itself, as Augustine declares [Enarr. ii in Ps. xxx, serm. 2; cf. Objection 1.
Yet this may happen to be sinful in four ways. First, if one have recourse to lots without any necessity: for this would seem to amount to tempting God. Hence Ambrose, commenting on the words of Luke 1:8, says: "He that is chosen by lot is not bound by the judgment of men." Secondly, if even in a case of necessity one were to have recourse to lots without reverence. Hence, on the Acts of the Apostles, Bede says (Super Act. Apost. i): "But if anyone, compelled by necessity, thinks that he ought, after the apostles' example, to consult God by casting lots, let him take note that the apostles themselves did not do so, except after calling together the assembly of the brethren and pouring forth prayer to God." Thirdly, if the Divine oracles be misapplied to earthly business. Hence Augustine says (ad inquisit. Januar. ii; Ep. lv): "Those who tell fortunes from the Gospel pages, though it is to be hoped that they do so rather than have recourse to consulting the demons, yet does this custom also displease me, that anyone should wish to apply the Divine oracles to worldly matters and to the vain things of this life." Fourthly, if anyone resort to the drawing of lots in ecclesiastical elections, which should be carried out by the inspiration of the Holy Ghost. Wherefore, as Bede says (Super Act. Apost. i): "Before Pentecost the ordination of Matthias was decided by lot," because as yet the fulness of the Holy Ghost was not yet poured forth into the Church: "whereas the same deacons were ordained not by lot but by the choice of the disciples." It is different with earthly honors, which are directed to the disposal of earthly things: in elections of this kind men frequently have recourse to lots, even as in the distribution of earthly possessions.
If, however, there be urgent necessity it is lawful to seek the divine judgment by casting lots, provided due reverence be observed. Hence Augustine says (Ep. ad Honor. ccxxviii), "If, at a time of persecution, the ministers of God do not agree as to which of them is to remain at his post lest all should flee, and which of them is to flee, lest all die and the Church be forsaken, should there be no other means of coming to an agreement, so far as I can see, they must be chosen by lot." Again he says (De Doctr. Christ. xxviii): "If thou aboundest in that which it behooves thee to give to him who hath not, and which cannot be given to two; should two come to you, neither of whom surpasses the other either in need or in some claim on thee, thou couldst not act more justly than in choosing by lot to whom thou shalt give that which thou canst not give to both."
This suffices for the Reply to the First and Second Objections.
Reply to Objection 3. The trial by hot iron or boiling water is directed to the investigation of someone's hidden sin, by means of something done by a man, and in this it agrees with the drawing of lots. But in so far as a miraculous result is expected from God, it surpasses the common generality of sortilege. Hence this kind of trial is rendered unlawful, both because it is directed to the judgment of the occult, which is reserved to the divine judgment, and because such like trials are not sanctioned by divine authority. Hence we read in a decree of Pope Stephen V [II, qu. v., can. Consuluist i]: "The sacred canons do not approve of extorting a confession from anyone by means of the trial by hot iron or boiling water, and no one must presume, by a superstitious innovation, to practice what is not sanctioned by the teaching of the holy fathers. For it is allowable that public crimes should be judged by our authority, after the culprit has made spontaneous confession, or when witnesses have been approved, with due regard to the fear of God; but hidden and unknown crimes must be left to Him Who alone knows the hearts of the children of men." The same would seem to apply to the law concerning duels, save that it approaches nearer to the common kind of sortilege, since no miraculous effect is expected thereupon, unless the combatants be very unequal in strength or skill.
Source: Summa Theologica (New Advent)