Note that persons under a sentence of
excommunication, associates and accomplices in the crime, notorious
evildoers and criminals, or servants giving evidence against their
masters, are admitted as witnesses in a case concerning the Faith.
And just as a heretic may give evidence against a heretic, so may a
witch against a witch; but this only in default of other proofs, and
such evidence can only be admitted for the prosecution and not for
the defence: this is true also of the evidence of the prisoner's
wife, sons and kindred; for the evidence of such has more weight in
proving a charge than in disproving it.
This is made clear in the c. in fidei de haer., where it says: As a protection of the faith we allow that in a case of inquiry into the sin of heresy, persons under excommunication and partners and accomplices in the crime shall be admitted as witnesses, in default of other proofs against heretics and their patrons, protectors and defenders; provided that it appears probably both from the number of the witnesses and of those against whom they give evidence, and from other cicumstances, that they are not giving false testimony.
The case of evidence given by perjurers, when it is presumed that they are speaking out of zeal for the faith, is deal with in the Canon c. accusatus, § licet, where it says that the evidence of perjurers, after they have repented, is admissable; and it goes on to say: If it manifestly appears that they do not speak in a spirit of levity, or from motives of enmity, or by reason of a bribe, but purely out of zeal for the orthodox faith, wishing to correct what they have said, or to reveal something about which they had kept silence, in defence of the faith, their testimony shell be as valid as that of anyone else, provided that there is no other obection to it.
And it is clear from the same chapter of the Canon that the testimony of men or low repute and criminals, and of servants against their masters, is admitted; for it says: So great is the plague of heresy that, in an action involving this crime, even servants are admitted as witnesses against their masters, and any criminal evildoer may give evidence against any person soever.