So what distinguishes the sacrament part from the valid marriage part? A priest and the witnesses? Are the witnesses especially ordained to the task of witnessing marriages or are they just Catholic friends of the bride and or groom?
Witnesses are necessary because marriage is a public act, a civil matter, as well as religious. It’s somewhat similar to having a last will and testament notarized, so there is sufficient evidence that the person signed it freely and of his own will, in case any doubt arises.
Marriage, among other things, concerns such matters as legitimacy of the children and rights of inheritance, so it has legal consequences. In an emergency, I believe two people can marry without witnesses or a priest, but they would want to formalize it at the first opportunity.
This, at least, was the case in earlier times. If the persons died now without formalizing it, it would be difficult to prove when it came to questions of inheritance, etc. But in the eyes of God it could be valid.
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