Article — Noldin, Summa of Moral Theology
Article Three. On Witnesses.
650. Who Must Assist, and How. In order that matrimony be valid, from the Tridentine decree and from the new Code at least two witnesses must assist at it.1 The new law requires no special quality in them.
All can be witnesses who can testify concerning something; therefore, for the validity of matrimony, it makes no difference whether the witnesses are men or women, faithful or infidels, blood relatives or outsiders, pubescent or impubescent, provided they have use of reason and can testify concerning the contracted matrimony. Nevertheless acatholic witnesses are used illicitly where Catholics can be had.2
In order that they can validly testify concerning the matrimony, the presence of all the witnesses (of the parish priest and of the others) is required to be a. physical, so that they can know what is being done; b. simultaneous, for since they must make faith concerning the same act, they must be present at it together; c. formal, that is, the contracting parties must have the intention of using their presence for celebrating matrimony, which intention they can sufficiently express by eyes, nod, etc., provided the witnesses notice this intention.3
Therefore even one who is present by chance, who is induced by fraud or fear to testify, can validly be a witness; this nevertheless does not have force concerning the parish priest.
1 Cn. 1094.
2 S. Office, August 19, 1891, established that "they are not to be used (heretics); nevertheless they can be tolerated by the ordinary from a grave cause, provided scandal is not present."
3 Ballerini-Palmieri VI, n. 859.