If a practicing Catholic is married in a Methodist Church to a Methodist with a Catholic Deacon “assisting” (not sure exactly what is meant by that), is it a valid marriage in the eyes of the Catholic Church?
Yes, if the following requirements are met….
Can. 1108 §1 Only those marriages are valid which are contracted in the presence of the local Ordinary or parish priest or of the priest or deacon delegated by either of them, who, in the presence of two witnesses, assists, in accordance however with the rules set out in the following canons, and without prejudice to the exceptions mentioned in canon 144, 1112 §1, 1116 and 1127 §23.
Can. 1115 Marriages are to be celebrated in the parish in which either of the contracting parties has a domicile or a quasidomicile or a month’s residence or, if there is question of vagi, in the parish in which they are actually residing. With the permission of the proper Ordinary or the proper parish priest, marriages may be celebrated elsewhere.
Can. 1118 §1 A marriage between Catholics, or between a catholic party and a baptized non-Catholic, is to be celebrated in the parish church. By permission of the local Ordinary or of the parish priest, it may be celebrated in another church or oratory.
§2 The local Ordinary can allow a marriage to be celebrated in another suitable place.
§3 A marriage between a catholic party and an unapprised party may be celebrated in a church or in another suitable place.
§3 It is forbidden to have, either before or after the canonical celebration in accordance with §1, another religious celebration of the same marriage for the purpose of giving or renewing matrimonial consent. Likewise, there is not to be a religious celebration in which the catholic assistant and a non-Catholic minister, each performing his own rite, ask for the consent of the parties.
In lay terms, this means……
"The ordinary minister at a mixed marriage is an authorized priest or deacon, and the ordinary place is the parish church of the Catholic party. A non-Catholic minister may not only attend the marriage ceremony but may also address, pray with, and bless the couple.
“For appropriate pastoral reasons, a bishop can grant a dispensation from the Catholic form of marriage and can permit the marriage to take place in a non-Catholic church with a non-Catholic minister as the officiating minister. A priest may not only attend such a ceremony but may also address, pray with, and bless the couple.”
‘It is not permitted,’ however, the bishops’ statement declared, ‘to have two religious services or to have a single service in which both the Catholic marriage ritual and a non-Catholic marriage ritual are celebrated jointly or successively’"
(1995 Catholic Almanac [Huntington, IN: Our Sunday Visitor, 1995], 233-4).