I know that a fallen away Catholic who marries outside of the Church would not have a valid marriage. What if a Catholic completely converts to Protestantism and then marries outside the Church? Is the marriage valid or does the fact that the person was Catholic at one time invalidate it?
The Church’s ecclesiastical law concerning The Form of the Celebration of Marriage is binding on all Catholics who have not formally defected from the Church. Code of Canon Law (CIC) states that the required form “must be observed if at least one of the parties contracting marriage was baptized in the Catholic Church or received into it and has not defected from it by a formal act” (CIC 1117).
Therefore, since a formally defected Catholic is not bound to the canonical form of marriage, his or her marriage outside the Church would generally be presumed to be valid, similar to a non-Catholic’s marriage outside the Church.
Related post: What does “Formal Defection” from the Church mean?