Dear Nathan:

This "promise" is called a vow. Canon Law 1193 states, "By its nature a vow obliges only the person who makes it." Thus, the vow made in your scenario is invalid as Jane is trying to obligate Mary. She cannot do that.

As for promises to God we learn from Canon Law 1191 that for the vow (promise) to be valid the following must be true (as explained by the University of Navarra commentary on Canon Law:

1) the person making the vow must be capable, the vow must be about something that is good, possible, and better than its opposite;

2) the object of the vow must be truly a promise, not a mere wish or intention, which implies that the person must have the intention of binding him or herself;

3) the promise must be made to God as the Supreme Lord of all creatures; that is, it must constitute an act of the virtue of religion

From these conditions, one may infer the defects that make a vow null and void:

1) lack of the use of reason or juridical capacity;

2) deceit, ignorance, error, violence, or fear when these are such as to prevent the subject (in view of his personal circumstances) from acting with sufficient knowledge and freedom;

3) the malice or impossibility (physical or moral) of the object;

4) the fact that the object is less perfect than its opposite;

5) the promise is made to a person other than God.

In your scenario to promise to make a pilgrimage is a good that is better than the opposite of not thanking God for Mary's successful surgery. If Jane truly intends to make the vow binding (on herself, not on Mary), and the vow is made to God, then the vow (promise) is valid and good.

But, making this promise cannot be an attempt to bargain with God and try to manipulate Him into causing Mary's surgery to be successful. Such a manipulation is putting God to the test and thereby is sinful.

The promise is not to be a test to see if God will do want Jane wants as if God needs Jane to take the pilgrimage and will not help Mary unless she does it. God does not "need" us to worship Him or to thank Him. Thus, offering this promise in this way is like a bribe. This will not work.

The promise, therefore, must be a thanksgiving to God if Mary's surgery is successful.

In any event, the promise made in your scenario is invalid on its face since Jane is trying to obligate Mary, which she cannot do.

God Bless,
Bro. Ignatius Mary