Objection 1. It would seem that a man's evidence ought not to be rejected except on account of some fault. For it a penalty on some that their evidence is inadmissible, as in the case of those who are branded with infamy. Now a penalty must not be inflicted save for a fault. Therefore it would seem that no man's evidence ought to be rejected save on account of a fault.
Objection 2. Further, "Good is to be presumed of every one, unless the contrary appear" [Cap. Dudum, de Praesumpt.]. Now it pertains to a man's goodness that he should give true evidence. Since therefore there can be no proof of the contrary, unless there be some fault of his, it would seem that no man's evidence should be rejected save for some fault.
Objection 3. Further, no man is rendered unfit for things necessary for salvation except by some sin. But it is necessary for salvation to give true evidence, as stated above (Article 1). Therefore no man should be excluded from giving evidence save for some fault.
On the contrary, Gregory says (Regist. xiii, 44): "As to the bishop who is said to have been accused by his servants, you are to know that they should by no means have been heard": which words are embodied in the Decretals II, qu. 1, can. Imprimis.
I answer that, As stated above (Article 2), the authority of evidence is not infallible but probable; and consequently the evidence for one side is weakened by whatever strengthens the probability of the other. Now the reliability of a person's evidence is weakened, sometimes indeed on account of some fault of his, as in the case of unbelievers and persons of evil repute, as well as those who are guilty of a public crime and who are not allowed even to accuse; sometimes, without any fault on his part, and this owing either to a defect in the reason, as in the case of children, imbeciles and women, or to personal feeling, as in the case of enemies, or persons united by family or household ties, or again owing to some external condition, as in the case of poor people, slaves, and those who are under authority, concerning whom it is to be presumed that they might easily be induced to give evidence against the truth.
Thus it is manifest that a person's evidence may be rejected either with or without some fault of his.
Reply to Objection 1. If a person is disqualified from giving evidence this is done as a precaution against false evidence rather than as a punishment. Hence the argument does not prove.
Reply to Objection 2. Good is to be presumed of everyone unless the contrary appear, provided this does not threaten injury to another: because, in that case, one ought to be careful not to believe everyone readily, according to 1 John 4:1: "Believe not every spirit."
Reply to Objection 3. To give evidence is necessary for salvation, provided the witness be competent, and the order of justice observed. Hence nothing hinders certain persons being excused from giving evidence, if they be considered unfit according to law.
Source: Summa Theologica (New Advent)