What happens a situation like this? The lapsed Catholic who doesn't believe can't take communion and can't make a valid confession. Does the lapsed Catholic have to formally defect for the marriage to be valid?
Here are the relevant canons. It requires permission from the Bishop for a Catholic to marry a lapsed Catholic:
*Can. 1071 §1. Except in a case of necessity, a person is not to assist without the permission of the local ordinary at:
4/ a marriage of a person who has notoriously rejected the Catholic faith;
§2. The local ordinary is not to grant permission to assist at the marriage of a person who has notoriously rejected the Catholic faith unless the norms mentioned in ⇒ can. 1125 have been observed with necessary adaptation.*
The canons on mixed marriage are to be observed to obtain permission. This includes assurance that the practicing Catholic is not in danger of defecting from the faith and that they promise to baptized and raise their children Catholic:
*Can. 1125 The local ordinary can grant a permission of this kind if there is a just and reasonable cause. He is not to grant it unless the following conditions have been fulfilled:
1/ the Catholic party is to declare that he or she is prepared to remove dangers of defecting from the faith and is to make a sincere promise to do all in his or her power so that all offspring are baptized and brought up in the Catholic Church;
2/ the other party is to be informed at an appropriate time about the promises which the Catholic party is to make, in such a way that it is certain that he or she is truly aware of the promise and obligation of the Catholic party;
3/ both parties are to be instructed about the purposes and essential properties of marriage which neither of the contracting parties is to exclude.*
Setting aside the Canons that *govern *such marriage, a Catholic should **seriously **rethink a plan that includes marrying a Catholic who has *rejected *the faith.