Deciding who goes to Hell or not is in God’s power alone, not ours or the Church’s. Yes, some who leave the Church will be damned and some will not, but no one can tell who will and why exactly. Leaving the Church will no doubt be a huge undoing on the part of a person and there are several reasons to back this up.
First, the Church is the Body of Christ so, leaving the Church means leaving the Body of Christ. The effect of leaving the Church can vividly be seen in John 15:5 where Christ likened Himself to the vine and we, the branches. Cut off from Him, we can do nothing. Our struggles in heavenly race will be futile because we can achieve nothing without Him.
Secondly, the Church is gifted with all tools necessary to attain God’s kingdom: The sacraments for graces, the Saints for emulation, the holy Eucharist, the Blessed Virgin Mary, the holy Mass and more. All these, when used wisely, can help one to attain God’s kingdom.
In Pope Pius XII’s encyclical Mystici corporis Christi of 1943, he says “Only those are to be accounted really members of the Church who have been regenerated in The Waters of baptism, professed the one true faith and have not cut themselves off from the structure of the Body by their own unhappy act or been severed therefrom for very grave crimes by the legitimate authority”. This means a real Catholic is identified by valid reception of baptism, profession of the Catholic faith and participation in the Communion of the Church.
Baptism leaves an indelible mark on a soul. So, no matter what sin one commits, the mark of baptism on one’s soul cannot be erased. Baptism is necessary for entering God’s kingdom (John 3:5).
For different reasons people leave the Church. Some were just habitual Catholics and not real Catholics which is why their faith could so easily have dwindled. Some, due to marital relationships with non-Catholics. Others leave to find the supposed “light” elsewhere which speaks volumes of their lack of knowledge of the holy Eucharist which is the true Body and Blood of Christ Himself, constantly present in the tabernacle. What other light could they possibly have been searching for?
However, those that are not aware that their actions are wrong remain connected to the Church spiritually although they won’t be regarded as legal members but those who are fully aware and believe that the Catholic Church is of true origin, nature and doctrine and still leave have no hope of being saved without repentance.
Whatever reason it may be for leaving the Church, one who leaves the Church is at risk of losing his salvation because the Catholic Church is the source of truth and graces and it is only a matter of time before a branch cut off from a tree starts to die off.