Dear Father, many years ago my brother, who was brought up Catholic albeit without much formation, married a Protestant and ever since has worshipped in a Protestant Church, where he is very active and convinced of his new beliefs. Can he still be saved?
At the outset we should remember that salvation is always a matter between an individual soul and God.
Whatever judgments we may make about a soul’s state and worthiness for salvation as a result of their decisions here on earth, in the end it is God who judges that soul and we should not presume to play God.
The Church has something to say about the matter in the Second Vatican Council’s Dogmatic Constitution on the Church Lumen gentium: “This holy Council first of all turns its attention to the Catholic faithful. Basing itself on scripture and tradition, it teaches that the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is mediator and the way of salvation; he is present to us in his body which is the Church. He himself explicitly asserted the necessity of faith and baptism (cf. Mk 16:16; Jn 3:5), and thereby affirmed at the same time the necessity of the Church, which men enter through baptism as through a door. Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it, or to remain in it” (LG 14).
The final words can sound harsh. They seem to say that someone who knows that the Catholic Church was founded as necessary for salvation and leaves it voluntarily in order to embrace some other faith cannot be saved.
Naturally these words do not apply to the many Catholics who no longer attend Mass, or who do so only irregularly, but still regard themselves as Catholics.
They have remained in the Church and form part of the family of the Church. Naturally, in order to be saved they must repent of their serious sins and be reconciled with God before they die.
Someone, on the contrary, who leaves the Church in order to embrace some other faith, be it Christian or non-Christian, has in fact refused to remain in the Church, so it would seem that in principle they cannot be saved.
Nonetheless, we would have to ask, and God would ask in the judgment, whether they really understood the importance of being Catholic, whether they knew that the Church was necessary for salvation.
As you say in your question, your brother did not have solid formation in the Catholic faith, so he could very well be in ignorance on this matter.
It is likely that most Catholics who leave the Church and embrace some other faith do not fully understand the importance of being Catholic.
When, like your brother, they embrace some other Christian faith, it may very well be that they think it really doesn’t matter to which denomination you belong, as long as it is Christian.
Most Christians do believe that salvation comes only through Christ, so these people are convinced that they are on the way to heaven, having taken Jesus as their Lord and Saviour.
The Second Vatican Council goes on to explain how the Catholic Church is related to these other Christians: “The Church knows that she is joined in many ways to the baptised who are honoured by the name of Christian, but who do not however profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter. For there are many who hold sacred scripture in honour as a rule of faith and of life, who have a sincere religious zeal, who lovingly believe in God the Father Almighty and in Christ, the Son of God and the Saviour, who are sealed by baptism which unites them to Christ, and who indeed recognise and receive other sacraments in their own Churches or ecclesiastical communities..; these Christians are indeed in some real way joined to us in the Holy Spirit…” (LG 15).
So in summary, we cannot judge whether any individual Catholic who leaves the Church and enters some other faith is saved.
It is up to God.
What we can do is pray very much for them that they may find the truth and live in accordance with it.
We can also speak with them about their situation and give them good reading matter.
By the way, “your brother” is my brother.