Objection 1. It seems that the priest should deny the body of Christ to the sinner seeking it. For Christ's precept is not to be set aside for the sake of avoiding scandal or on account of infamy to anyone. But (Matthew 7:6) our Lord gave this command: "Give not that which is holy to dogs." Now it is especially casting holy things to dogs to give this sacrament to sinners. Therefore, neither on account of avoiding scandal or infamy should this sacrament be administered to the sinner who asks for it.
Objection 2. Further, one must choose the lesser of two evils. But it seems to be the lesser evil if the sinner incur infamy; or if an unconsecrated host be given to him; than for him to sin mortally by receiving the body of Christ. Consequently, it seems that the course to be adopted is either that the sinner seeking the body of Christ be exposed to infamy, or that an unconsecrated host be given to him.
Objection 3. Further, the body of Christ is sometimes given to those suspected of crime in order to put them to proof. Because we read in the Decretals: "It often happens that thefts are perpetrated in monasteries of monks; wherefore we command that when the brethren have to exonerate themselves of such acts, that the abbot shall celebrate Mass, or someone else deputed by him, in the presence of the community; and so, when the Mass is over, all shall communicate under these words: 'May the body of Christ prove thee today.'" And further on: "If any evil deed be imputed to a bishop or priest, for each charge he must say Mass and communicate, and show that he is innocent of each act imputed." But secret sinners must not be disclosed, for, once the blush of shame is set aside, they will indulge the more in sin, as Augustine says (De Verbis. Dom.; cf. Serm. lxxxii). Consequently, Christ's body is not to be given to occult sinners, even if they ask for it.
On the contrary, on Psalm 21:30: "All the fat ones of the earth have eaten and have adored," Augustine says: "Let not the dispenser hinder the fat ones of the earth," i.e. sinners, "from eating at the table of the Lord."
I answer that, A distinction must be made among sinners: some are secret; others are notorious, either from evidence of the fact, as public usurers, or public robbers, or from being denounced as evil men by some ecclesiastical or civil tribunal. Therefore Holy Communion ought not to be given to open sinners when they ask for it. Hence Cyprian writes to someone (Ep. lxi): "You were so kind as to consider that I ought to be consulted regarding actors, end that magician who continues to practice his disgraceful arts among you; as to whether I thought that Holy Communion ought to be given to such with the other Christians. I think that it is beseeming neither the Divine majesty, nor Christian discipline, for the Church's modesty and honor to be defiled by such shameful and infamous contagion."
But if they be not open sinners, but occult, the Holy Communion should not be denied them if they ask for it. For since every Christian, from the fact that he is baptized, is admitted to the Lord's table, he may not be robbed of his right, except from some open cause. Hence on 1 Corinthians 5:11, "If he who is called a brother among you," etc., Augustine's gloss remarks: "We cannot inhibit any person from Communion, except he has openly confessed, or has been named and convicted by some ecclesiastical or lay tribunal." Nevertheless a priest who has knowledge of the crime can privately warn the secret sinner, or warn all openly in public, from approaching the Lord's table, until they have repented of their sins and have been reconciled to the Church; because after repentance and reconciliation, Communion must not be refused even to public sinners, especially in the hour of death. Hence in the (3rd) Council of Carthage (Can. xxxv) we read: "Reconciliation is not to be denied to stage-players or actors, or others of the sort, or to apostates, after their conversion to God."
Reply to Objection 1. Holy things are forbidden to be given to dogs, that is, to notorious sinners: whereas hidden deeds may not be published, but are to be left to the Divine judgment.
Reply to Objection 2. Although it is worse for the secret sinner to sin mortally in taking the body of Christ, rather than be defamed, nevertheless for the priest administering the body of Christ it is worse to commit mortal sin by unjustly defaming the hidden sinner than that the sinner should sin mortally; because no one ought to commit mortal sin in order to keep another out of mortal sin. Hence Augustine says (Quæst. super Gen. 42): "It is a most dangerous exchange, for us to do evil lest another perpetrate a greater evil." But the secret sinner ought rather to prefer infamy than approach the Lord's table unworthily.
Yet by no means should an unconsecrated host be given in place of a consecrated one; because the priest by so doing, so far as he is concerned, makes others, either the bystanders or the communicant, commit idolatry by believing that it is a consecrated host; because, as Augustine says on Psalm 98:5: "Let no one eat Christ's flesh, except he first adore it." Hence in the Decretals (Extra, De Celeb. Miss., Ch. De Homine) it is said: "Although he who reputes himself unworthy of the Sacrament, through consciousness of his sin, sins gravely, if he receive; still he seems to offend more deeply who deceitfully has presumed to simulate it."
Reply to Objection 3. Those decrees were abolished by contrary enactments of Roman Pontiffs: because Pope Stephen V writes as follows: "The Sacred Canons do not allow of a confession being extorted from any person by trial made by burning iron or boiling water; it belongs to our government to judge of public crimes committed, and that by means of confession made spontaneously, or by proof of witnesses: but private and unknown crimes are to be left to Him Who alone knows the hearts of the sons of men." And the same is found in the Decretals (Extra, De Purgationibus, Ch. Ex tuarum). Because in all such practices there seems to be a tempting of God; hence such things cannot be done without sin. And it would seem graver still if anyone were to incur judgment of death through this sacrament, which was instituted as a means of salvation. Consequently, the body of Christ should never be given to anyone suspected of crime, as by way of examination.
Source: Summa Theologica (New Advent)