Losing salvation through false beliefs?

I have a niece who will be marrying a Catholic soon and am concerned about his spiritual life. We know that Catholics are baptized as infants also, and receive faith in their Savior as a result. We also know that it is not necessary to get baptized again, if they become Lutheran. From here, it gets 'fuzzy' as far as what the Roman Catholics believe and whether they are saved or not. If a Catholic, through the church's teaching, was led to believe that he is required to do 'good works' in order to maintain salvation, this is indeed a false teaching, yet he received the same Holy Spirit we received at baptism. Then, it is the church's teaching that is 'corrupt' and I guess my question is, "Is this a way for a person to 'lose' his salvation, through his own false beliefs? Is this a good example of having our own 'will' in deciding what is true or false? What if a person believes this teaching out of ignorance? I would appreciate it if you could clarify this. I do know that God is the judge of all and he only can read our hearts, and whether we are truly 'dependent on Him.'

You have a correct understanding:  the Holy Spirit can plant saving faith in a person’s heart, but then that faith can become corrupted by a person believing the false teachings of a church or believing the false teachings that arise from the person’s own thinking.  While ignorance of Scripture may explain why a person has a misguided faith, it does not exempt the person from being accountable to God.

All this is reason why you and I need to keep sharing the truths of Scripture with others.  “For we maintain that a person is justified by faith apart from the works of the law” (Romans 3:28).  “For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast” (Ephesians 2:8-9).  God bless your ongoing testimony!