What does a returning Catholic who contracted marriage in a Protestant church need to do when the spouse wants to remain Protestant?

I will preface this by stating I am not a canon lawyer, so I am not an expert on canon law and my answer should not be taken to be authoritative. However, I am offering an interpretation of canon law that I think is proper.

First of all, the marriage is not valid. Ordinarily, two people not in the Catholic Church could contract a valid, dissoluble (ie divorce is possible) natural marriage outside of the Catholic Church. Ordinarily, baptized Christians who were never baptized nor confirmed in the Catholic Church could contract a valid, indissoluble (ie divorce is not possible) sacramental marriage outside of the Catholic Church. Ordinarily, if either party to the marriage is baptized and/or confirmed as a Catholic, a valid marriage, natural or sacramental, cannot be obtained outside of the Catholic Church, that is, outside of the prescribed canonical form for a Catholic marriage. This is ordinarily true, but bishops may grant dispensations to have a Catholic marriage validly contracted outside of the prescribed canonical form. In your proposed scenario, it would seem that no such dispensation was sought nor granted, thus we must conclude the marriage is ipso facto invalid. I doubt a tribunal would seek any further evidence to annul this marriage, since questions of consent and the like are irrelevant in this case.

That being said, your hypothetical Catholic wants to keep his or her marriage in tact. Assuming that the protestant party also desires this, radical sanation does seems like the proper route to go. The Code of Canon Law regarding radical sanation is as follows:

RADICAL SANATION

Can. 1161 §1. The radical sanation of an invalid marriage is its convalidation without the renewal of consent, which is granted by competent authority and entails the dispensation from an impediment, if there is one, and from canonical form, if it was not observed, and the retroactivity of canonical effects.

§2. Convalidation occurs at the moment of the granting of the favor. Retroactivity, however, is understood to extend to the moment of the celebration of the marriage unless other provision is expressly made.

§3. A radical sanation is not to be granted unless it is probable that the parties wish to persevere in conjugal life.

Can. 1162 §1. A marriage cannot be radically sanated if consent is lacking in either or both of the parties, whether the consent was lacking from the beginning or, though present in the beginning, was revoked afterwards.

§2. If this consent was indeed lacking from the beginning but was given afterwards, the sanation can be granted from the moment the consent was given.

Can. 1163 §1. A marriage which is invalid because of an impediment or a defect of legitimate form can be sanated provided that the consent of each party perseveres.

§2. A marriage which is invalid because of an impediment of natural law or of divine positive law can be sanated only after the impediment has ceased.

Can. 1164 A sanation can be granted validly even if either or both of the parties do not know of it; nevertheless, it is not to be granted except for a grave cause.

Can. 1165 §1. The Apostolic See can grant a radical sanation.

§2. The diocesan bishop can grant a radical sanation in individual cases even if there are several reasons for nullity in the same marriage, after the conditions mentioned in can. 1125 for the sanation of a mixed marriage have been ful-filled. He cannot grant one, however, if there is an impediment whose dispensation is reserved to the Apostolic See according to the norm of can. 1078, §2, or if it concerns an impediment of natural law or divine positive law which has now ceased.

Canon 1078 carves out two impediments that are reserved to the Holy See, namely, those regarding Holy Orders and those regarding a crime regarding canon 1090. I am assuming your hypothetical Catholic spouse never received Holy Orders and is not guilty of any impeding crimes, so I am concluding that the diocesan bishop can grant the sanation.

Googling Radical Sanation first yielded a parish website from Texas regarding the practice. This site outlines three requirements:

A. Both parties must have the intent of persevering in their conjugal life. Otherwise a "radical sanation" cannot be granted.

I am assuming that the both spouses want the relationship to continue qua marriage.

B. Both parties (a) must have consented to the marriage (b) at the same time. This simultaneous consent had to include all the essential requirements of a valid marriage (e.g. monogamy, fidelity, permanence and openness to children). This consent must still exist at present unless one party has indicated otherwise.

We can probably assume that both parties consented at least to monogamy, fidelity, and permanence at the time of the marriage, since they were married in a Protestant church and with a Protestant marital theology. They also may have both been open to children, or one or both of them may not have been. If they were not then, but later became open to children while still consenting to the other three requirements, and still do consent to all four requirements at the time that radical sanation is sought, then they can receive radical sanation retroactively, from the moment when they both consented to all four requirements of marriage. Neither Canon Law nor this website stipulate that consent to raising children in the faith is required of both parties. No ceremony is required, so it would appear to me that an interview of both parties gathering all four kinds of consent on the part of the bishop or an appointed representative should be sufficient to grant radical sanation and retroactively validate the marriage.

C. Any impediments that are present, they must be taken care of. In many cases, such is achieved by obtaining a "radical sanation." When marrying outside the Catholic Church, such as before a Justice of the Peace, the marriage was invalid because of a "Defect of Form." That means that the parties failed to obtain a dispensation from the Bishop for a marriage ceremony outside of the Catholic Church.

I'm assuming the only impediment here is a defect of form, which the radical sanation is supposed to rectify.

In Conclusion

It seems to me that even without the protestant spouse consenting specifically to raising the children Catholic, radical sanation is theoretically possible, since neither Canon Law nor the parish site I found offering to help Catholics obtain it mentioned this requirement. However, practically, your bishop may not be inclined to grant it without such consent, as this refusal will cause the marriage to become very difficult, and he will be responsible for unequally yoking a believer to a non-believer, without even the consent from the non-believer that the children would be raised in the faith. In that case, the Catholic spouse must refrain from marital relations, since the marriage is not valid and any such relation is fornication. Seeking annulment might be the best route if your bishop will not grant radical sanation due to your partner's refusal to raise children specifically in the Catholic faith. If you think you both can bear it, living "as brother and sister" is an option for the sake of your children having a mother and father in the home.