Q: I have been asked in preparing a couple for their upcoming wedding if it would be allowed that they could recite vows to one another that they themselves have written, either in addition to or in replace of the Traditional Marriage vows?
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A: They need to recite one of the approved formulas, which constitute the form of the sacrament of marriage. They don’t want to recite words that would later cause a challenge to the validity of their marriage.
I suggest you ask a canon lawyer for another opinion, but I advise against it.