Leaving the Church would, according to Catholic teaching, definitely be a grave sin (schism).
The Church will not say that a given individual has committed a “mortal” sin, because that implies knowledge and intent. A person who genuinely believes that they are following God in leaving the Church might well be in a state of grace even while committing the objectively grave sin of schism.
Edwin
Here is what the bible foretells for ALL those who for any reason leave the RCC
Heb.6: 1 to 8
"Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, with instruction about ablutions, the laying on of hands, the resurrection of the dead, and eternal judgment. And this we will do if God permits.** For it is impossible to restore again to repentance those who have once been enlightened [BAPTIZED AND RECEIVED THE GRACES[/COLOR], who have tasted the heavenly gift,[CATHOLIC HOLY COMMUNION:JESUS HIMSELF] and have become partakers of the Holy Spirit,[BEEN CONFIRMED IN THE RCC] and have tasted the goodness of the word of God [BEEN EXPOSED TO CHRIST TRUTHS] and the powers of the age to come,
if they then commit apostasy, since they crucify the Son of God on their own account and hold him up to contempt. For land which has drunk the rain that often falls upon it, and brings forth vegetation useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed; its end is to be burned.
And here is what the Catechism teaches:
1260 “Since Christ died for all, and since all men are in fact called to one and the same destiny, which is divine, we must hold that the Holy Spirit offers to all the possibility of being made partakers, in a way known to God, of the Paschal mystery.” Every man who is ignorant of the Gospel of Christ and of his Church, but seeks the truth and does the will of God in accordance with his understanding of it, can be saved. It may be supposed that such persons would have desired Baptism explicitly if they had known its necessity.
846 How are we to understand this affirmation, often repeated by the Church Fathers? Re-formulated positively, it means that all salvation comes from Christ the Head through the Church which is his Body:
Basing itself on Scripture and Tradition, the Council teaches that the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is the 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, 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
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.
848 “Although in ways known to himself God can lead those who, through no fault of their own, are ignorant of the Gospel, to that faith without which it is impossible to please him, the Church still has the obligation and also the sacred right to evangelize all men.”
AMEN
GBY**
