Objection 1. It would seem that it is not unlawful to form a judgment from suspicions. For suspicion is seemingly an uncertain opinion about an evil, wherefore the Philosopher states (Ethic. vi, 3) that suspicion is about both the true and the false. Now it is impossible to have any but an uncertain opinion about contingent singulars. Since then human judgment is about human acts, which are about singular and contingent matters, it seems that no judgment would be lawful, if it were not lawful to judge from suspicions.
Objection 2. Further, a man does his neighbor an injury by judging him unlawfully. But an evil suspicion consists in nothing more than a man's opinion, and consequently does not seem to pertain to the injury of another man. Therefore judgment based on suspicion is not unlawful.
Objection 3. Further, if it is unlawful, it must needs be reducible to an injustice, since judgment is an act of justice, as stated above (Article 1). Now an injustice is always a mortal sin according to its genus, as stated above (Question 59, Article 4). Therefore a judgment based on suspicion would always be a mortal sin, if it were unlawful. But this is false, because "we cannot avoid suspicions," according to a gloss of Augustine (Tract. xc in Joan.) on 1 Corinthians 4:5, "Judge not before the time." Therefore a judgment based on suspicion would seem not to be unlawful.
On the contrary, Chrysostom [Hom. xvii in Matth. in the Opus Imperfectum falsely ascribed to St. John of the Cross] in comment on the words of Matthew 7:1, "Judge not," etc., says: "By this commandment our Lord does not forbid Christians to reprove others from kindly motives, but that Christian should despise Christian by boasting his own righteousness, by hating and condemning others for the most part on mere suspicion."
I answer that, As Tully says (De Invent. Rhet. ii), suspicion denotes evil thinking based on slight indications, and this is due to three causes. First, from a man being evil in himself, and from this very fact, as though conscious of his own wickedness, he is prone to think evil of others, according to Ecclesiastes 10:3, "The fool when he walketh in the way, whereas he himself is a fool, esteemeth all men fools." Secondly, this is due to a man being ill-disposed towards another: for when a man hates or despises another, or is angry with or envious of him, he is led by slight indications to think evil of him, because everyone easily believes what he desires. Thirdly, this is due to long experience: wherefore the Philosopher says (Rhet. ii, 13) that "old people are very suspicious, for they have often experienced the faults of others." The first two causes of suspicion evidently connote perversity of the affections, while the third diminishes the nature of suspicion, in as much as experience leads to certainty which is contrary to the nature of suspicion. Consequently suspicion denotes a certain amount of vice, and the further it goes, the more vicious it is.
Now there are three degrees of suspicion. The first degree is when a man begins to doubt of another's goodness from slight indications. This is a venial and a light sin; for "it belongs to human temptation without which no man can go through this life," according to a gloss on 1 Corinthians 4:5, "Judge not before the time." The second degree is when a man, from slight indications, esteems another man's wickedness as certain. This is a mortal sin, if it be about a grave matter, since it cannot be without contempt of one's neighbor. Hence the same gloss goes on to say: "If then we cannot avoid suspicions, because we are human, we must nevertheless restrain our judgment, and refrain from forming a definite and fixed opinion." The third degree is when a judge goes so far as to condemn a man on suspicion: this pertains directly to injustice, and consequently is a mortal sin.
Reply to Objection 1. Some kind of certainty is found in human acts, not indeed the certainty of a demonstration, but such as is befitting the matter in point, for instance when a thing is proved by suitable witnesses.
Reply to Objection 2. From the very fact that a man thinks evil of another without sufficient cause, he despises him unduly, and therefore does him an injury.
Reply to Objection 3. Since justice and injustice are about external operations, as stated above (58, 8,10,11; 59, 1, ad 3), the judgment of suspicion pertains directly to injustice when it is betrayed by external action, and then it is a mortal sin, as stated above. The internal judgment pertains to justice, in so far as it is related to the external judgment, even as the internal to the external act, for instance as desire is related to fornication, or anger to murder.
Source: Summa Theologica (New Advent)