As we explained last week, Jewish Law requires the presence of two witnesses in a marriage ceremony (see here). As a general rule, no single witness alone is competent to attest or testify.
Not anyone is competent to act as a witness.
Some examples:
In Jewish Law, a person is incompetent as a witness until he reaches the age of 13. In some cases, real estate for example, the minimum age required is 20 (MT, Edut 9:6).
The wicked (resha'im) are incompetent witnesses. This includes: criminals, swindlers, perjurers and informers; persons who have committed capital offenses, thieves and robbers, usurers, tricksters, gamblers and gamesters, as well as idlers or vagabonds who are suspected of spending their leisure in criminal activities (see more details in Shulḥan 'arukh Ḥoshen Mishpat Chapter 34).
A man who has no basic knowledge of Tora (Bible) or Mishna, nor of civilized standards of conduct (derekh ereṣ), is presumed to be idle and disorderly and therefore is considered to be incompetent as a witness. This presumption, however, is rebuttable by evidence that, notwithstanding the man's illiteracy, his conduct is irreproachable.
A person called to attest or testify together with another person whom he knows to be incompetent as a witness must decline to testify since the incompetence of any one witness invalidates the testimony of the whole group of witnesses. This is why in a marriage ceremony the Rabbi will carefully choose two witnesses and indicate to the groom to appoint them "to the exclusion of all others", i.e., other people who are incidentally witnessing the ceremony and who might not qualify as witnesses.
(To be continued...).
READ: Ed Koch & Rabbi Yisrael Meir Lau "I'm a Holocaust survivor too." By Ivette Alt Miller, from Aish.com