If a person leaves the church and joins another faith for 10 plus years, what is required to return to good standing in the church?
This isn’t as hard as one might imagine.
Usually all a person has to do is make a sincere and complete confession of all mortal sins of commission or omission since the last confession, have whatever censures she incurred lifted (not hard), and then return to the proper life of a good Catholic! Easy Peasy.
In the case that a person made a formal act of apostasy (e.g., in Germany in order to avoid paying the Church tax), she – once upon a time – would have had to go though other steps before returning to the sacraments. However, in 2009 a document called Omnium in mentem was issued whereby the Church’s law about these formal acts was changed. Now, the Church no longer considers it possible to defect from the faith by formal act. Therefore, there are no canonical consequences from formal defection. Were a person to film herself signing a document and then publish the photos and take out ads in the newspaper, according to the Church they would not have formally defected from the Church.
Thus, people cannot now formally defect. They can, however, still incur a censure of excommunication – a spiritual and medicinal penalty – for heresy or schism or apostasy (cf. can 1364). In order to incur any censure she would have had to understand the consequences of the act. Therefore, if she joined another church without really understanding the canonical consequences (e.g., she married a Lutheran and started going to services with her spouse and then joins the Lutheran parish…) then it is likely that no excommunication is incurred.
So, in order to return to the light and grace of the Church and leave the darkness of soul endangering heresy and schism, in most cases simply going to confession and getting any censure lifted is all one has to do to.
Finally, on a slightly different but related note, the Code says in can. 1366 that parents or those who take the place of parents who have their children baptized or educated in a non-Catholic religion are to be punished with a censure or other just penalty. If we have an obligation to maintain our Catholic identity for ourselves, we also have an obligation to maintain the Catholic identity of children for whom we are responsible.