Is a Catholic who converts to another Christian faith damned?
As the Catechism of the Catholic Church teaches CCC 818:
However, one cannot charge with the sin of the separation those who at present are born into these communities [that resulted from such separation] and in them are brought up in the faith of Christ, and the Catholic Church accepts them with respect and affection as brothers . . . . All who have been justified by faith in Baptism are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers in the Lord by the children of the Catholic Church.Ah, but does this apply to someone who was either was baptized as a baby and raised in the faith, or converted to Catholicism at a later time and then leaves the Church?
I would say rather that CCC 2088 and 2089 apply:
2088 The first commandment requires us to nourish and protect our faith with prudence and vigilance, and to reject everything that is opposed to it. There are various ways of sinning against faith: Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief. Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated doubt can lead to spiritual blindness.This section of the CCC concerns sins against the faith. To be clear such sins are mortal sins. What is a mortal sin?
2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him."
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."What does the Catechism say about someone who dies under the burden of mortal sin? (CCC1861)
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.Someone I know contends that many who have left the Church have done so because they were never properly catechized. That is they do not have full knowledge and so cannot be in a state of mortal sin.
A case could also be made that a child who has been baptized, but is subsequently moved by their parents to another faith community lacks, for this purpose the ability to refuse to change denominations because they do not have the ability to exercise or withhold deliberate consent in this act.
They further point out that (CCC 847):
This affirmation is not aimed at those who, through no fault of their own, do not know Christ and his Church:
Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience - those too may achieve eternal salvation.
This is what I call the NBA clause.
Every American boy of a certain social economic class would like to believe that if they can play basketball well enough, that even though they are not stellar academic performers, nor do they have the social or political connections to become economically successful through other means, they can be the next Michael Jordan.
Of course the truth is that most will never be NBA players and they would be much better spending their time studying math than playing basketball.
So while the atheistic or agnostic equivalent of Mother Teresa might have a shot at heaven I would say that most will have as much chance as your typical inner city youth at making the NBA.
That being the case, while, as CCC says we must "entrust judgment of persons to the justice and mercy of God.," it is not helpful in my opinion to sugarcoat the matter.
In the time since Vactican II (indeed since and even before the Reformation) there have been those who, guilty of the sins of Incredulity, Heresy, apostasy, and schism, have confused the faithful and besides placing their own souls in jeopardy have also placed the souls of others in peril. Some of these have been priest, some members of the laity, some even bishops or theologians. Most cannot be considered to have been "badly catechized" or to have lacked the power of deliberate consent.
Their fate lies with God alone, but I wouldn't take odds on their final destination, and I don't see how lying to others on their probable fate is in those other's spiritual best interests