If one abandons his faith in Catholicism and becomes an Atheist, could he still go to heaven?

Actually, you don't "Convert" to atheism. It's a belief or worldview, that denies God, and also heaven and hell.

Think about it for a minute.

If there is no God, then there is no absolute truth, and no sovereign divine Judge who is keeping a record and there is no absolute standard for morality - [the Bible] and heaven and hell are meaningless.

People are atheists, not because there is a lack of evidence for a divine creator, but at the end of the day, they don't want to be accountable for their actions, and so it's easier to just deny God, but any intelligent person would laugh at the notion that a ticking watch in the forest just came about by random chance, or came about from an explosion.

Design implies a designer. Even a mousetrap with 5 parts could not create itself, and illustrates the concept of "irreducible complexity' - if you just took away any one of the 5 simple parts of a mouse trap, then it's no longer mouse trap. All the parts have to be present and working together. Being an atheist is not logical.

The biggest and most important issue is that you seem to hold the erroneous view that being a Catholic means you "already are going to heaven"?

Being Protestant or Baptist or Catholic does not automatically make you born again.

There are millions of "religious people" who do the motions outwardly, but they don't have a relationship with Christ, and Christ himself said of the religious leaders who prided themselves in doing good works and keeping the law - that they were like "white-washed graves, but inside, they are like dead mens' bones."

Going to church, and doing good works, or giving money to the poor means nothing if we are not born again, and if we have not repented of our sins and asked Christ to forgive us. This is true for every person whether Catholic or Protestant.

• "Jesus said, I am the way, the truth and the life. No one comes to the Father, but by me. John 14:6
• John 3:3 "Jesus responded and said to him, “Truly, truly, I say to you, unless someone is born again he cannot see the kingdom of God.”
• Romans 10:9-10 9 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. 10 For with the heart one believes and is justified, and with the mouth one confesses and is saved.
• I John 1:9 Berean Study Bible If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.

Yes he can.

God loves us, and by redeemed, meaning that we need to be united in one baptism, be it a legal, or even a baptism united only by heart/intention AND GOOD DOINGS IN LIFE, one is considered having no knowledge deep enough about God's love and truth, because if he understand and feel it, he will love and acknowledge God. A sin is mortal if it fulfilled 3 conditions, grave matters, full knowledge, full control... So he might not sin gravely if he (or his logic) didn't (or unable to) know the truth. (in fact, many people who weren't christians managed a good holy life, Muslims, Hindus, Buddhas, even Atheists... Catholicism is a VIP benefit to live a holy life, it is NOT A GUARANTEE benefit)

Atheists, especially clever men which are at least master degree quantum physics (i have some uncles and cousins, who are S2 and S3 of physics), acknowledge "something/superior intelligence" which is needed to make the Big Bang happen and grow human... By variables of carbons, water, electricity, etc, in which only having tolerance of failure of 0.0000**1%, to make the creation happen. The superior intelligence made all the variables met. But they don't want to name the superior intelligence by "God"... Imagine a boy who hates his dad for not buying him weeds, and try to call his dad "uncle".

Note: my confession father says that faith doesn't have to contradict science, philosophy (and faith) WAS ONE with science... in fact the Church encourages scientists to strengthen their faith by heart AND LOGIC.

Fear shouldn't drive your actions

There are many reasons to believe or not believe and each is personal - that isn't the debate here. It sounds like you've made your mind up and have realised that you don't believe in a God anymore.

Your life so far has likely included many days in sunday school highlighting the importance of following Catholicism and the after-life consequences of not doing so. The moral choices you make are much more important than the religious ones.

Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. ~ Matt. 7:21

In a letter to an atheist Pope Francis says:

As for the three questions you asked me in the article of August 7th. It would seem to me that in the first two, what you are most interested in is understanding the Church's attitude towards those who do not share faith in Jesus. First of all, you ask if the God of the Christians forgives those who do not believe and do not seek faith. Given that - and this is fundamental - God's mercy has no limits if he who asks for mercy does so in contrition and with a sincere heart, the issue for those who do not believe in God is in obeying their own conscience. In fact, listening and obeying it, means deciding about what is perceived to be good or to be evil. The goodness or the wickedness of our behavior depends on this decision.

Whether you choose Atheism or Catholicism (or another religion) the most important thing is that you choose to be a good person ("obey [your] own conscience"). Don't make that choice because you desire heaven and fear hell, make it because you want to be that person in your time here on earth.

The goodness or wickedness of a person depends on this, not their faith or lack of it.

Personal opinion:

Your life and belief is a personal journey and beyond all else honesty is most important. This can be viewed through either lens. If there is a God then they, too, know what you're really feeling, hiding it and pretending going through the motions isn't fooling them - only those around you. If there isn't a God then you'd be following for no reason.

There is no point in pretense - only in finding what you believe to be true. Talk to people on all sides, explore your doubts - search for different opinions on your faith. Perhaps what doesn't sit right with you is an issue resolved in another sect of Christianity or even another religion. Perhaps Atheism is where the world will make sense to you.

The most important thing is to be a good person, regardless of what you end up believing.

Introduction (personal opinion)

Before presenting the Catholic Church's teaching on abandoning a person's childhood Catholic faith, I would like to review the connection of all elements in your question: God, heaven, the Catholic church, Atheism, the meaning of Catholic practice, and the fear of not going to heaven if Atheism turns out to be false.

God, as understood by Christians, is a good and loving Father who wants to save us from the power of sin by sending His Son Jesus to die on the cross for us as a sacrifice of love, a tangible sign so we can believe His ultimate intention for us: to enter heaven. That is the Gospel.

So God invites us to live in His house: heaven. But only people with hearts that fit for heaven can enter since heaven is the place for love. It makes sense, because to flourish in heaven we need to have the hearts that have the same character as God: loving. Do you think entering heaven is automatic? Are our hearts ready to enter heaven today? Since God gives us free will, we have a choice on how to shape our hearts:

1. To the Christian standard, the way God created us to be
2. To the standard that we create for ourselves, because that's what Atheism basically is.

We also need to be aware that there is an enemy! It's not just us and God. This enemy is Satan and his minions, the one who tempts us to sin, the one who tirelessly tries to prevent us from entering heaven through lies such as false religions and false belief systems. Satan is the prince of the world for now until he is completely defeated at the end of the world. We now live in enemy occupied territory. Satan's main mission is to prevent us from shaping our hearts to God's standard by enticing us to create our own standard, like Atheism!

Can Atheism prepare our hearts for the Christian heaven? Remember, heaven is the Christian God's house. But Atheism means believing that God doesn't exist. If through living our lives we shape our hearts as though God doesn't exist, it's like living in the same house with our parents but we:

• never say anything to them
• never ask them what we can do to please them, and
• set our own rules while living in THEIR house!

A heart that habitually ignores God instead of loving God. Does it fit? It simply doesn't fit !!

The only fitting choice is to prepare our hearts according to Christianity, God's standard, the owner of heaven. That is why God establishes the Catholic Church on earth, a stronghold within the enemy occupied territory. The Catholic Church's main mission is to help us prepare our hearts for heaven in the following ways:

• reconcile people to God (baptism, confession)
• teach the truths about God (catechism)
• nourish the faithful spiritually (mass, prayer, eucharist)
• help us to be more loving (church life)

The teaching of the Catholic Church (presented below) is meant for the discernment of our true state of heart, to help us discover the sins of our hearts and to help us grow in our love for God and our neighbors. It is not the Church who sends a person to heaven or to hell; the Church is merely a reliable guide, a "truth telling thing" as the great Catholic apologist G.K. Chesterton put it, to counter Satan, the Father of Lies. The Catholic priests help administer the sacraments throughout our earthly journey to heaven: Baptism, Confirmation, Communion & Confession, Marriage / Ordination (optional), and Last Rites (a combination of Confession + Anointing of the sick + final Communion) just before we die. If our destination is the Christian heaven, what's the point of leaving the Catholic Church?

Finally, let's talk about fear. There is a bad fear and a good fear. Bad fear is doing things only because we don't want to be punished. We do them simply because other people (like our parents) tell us to do them. Or we do them without love and without understanding why we do them. Bad fear is a fear of servitude or slavery, which God wants to turn into a good fear. Good fear is respect and gratitude for God expressed in good actions dedicated as an offering to Him. We are fearful for going astray because this will grieve God's heart who wants us to safely complete our earthly journey to enter His home: heaven. It's like being fearful to hurt people who love us, like our parents, who only want good things for us. Good fear also means freely choosing and submitting to the standard that God sets for us (Christianity), because the standard will sets us more free to love Him and leads us to a more meaningful life, on earth as well as in heaven.

Teaching of the Catholic Church

From your description, since the person seems to be

• a baptized Catholic who
• voluntarily (without external threat) and
• knowingly (after consulting parents, who I presume would guide him/her in the Catholic Faith, and therefore is no longer ignorant; whether partially / fully ignorant it's hard to determine)
• leaves the Church and
• joins the creed of Atheism

then from the point of view of the Catholic Church the person has committed at least a material sin of heresy which could develop into the formal variety, and even worse, to the sin of apostasy (who leaves not just the Catholic Church but abandons Christ Himself!).

If either heresy or apostasy are committed with full knowledge and full intentionality (which is hard to determine because we cannot see into the person's heart completely) then it is a grave matter and therefore a mortal sin, and thus incurs automatic latae sententiae excommunication. Excommunication does not mean that the Catholic church sends a person to hell (see good explanation here). One must also satisfy the general conditions for canonical culpability set out in Canon Law Code 1321-1323. But if this person's act of becoming an atheist can with certainty be classified as a mortal sin, this person will not go to heaven.

About your subquestion "if God was real, could they still go to heaven", this reminds me of Pascal's wager that "humans bet with their lives that God either exists or does not". Adapted to this context:

• If the Christian God exists then the above criteria of entering heaven applies
• If the Christian God does NOT exist, that means there is no Christian heaven to enter either

The bottom line (quoted from ChurchPOP website article Abandoning Christ: The Forgotten Sins of Heresy, Apostasy, and Schism):

If you are a baptized Christian, you should hold fast to the teachings of the Church, keep practicing the faith, and maintain unity with your bishop and especially the Pope.

Doing anything less is abandoning Christ and his Church!

My personal advice:

• Consult the Catholics you can trust, like your priest, deacon, catechist / RCIA team, or knowledgeable members of the parish.
• Bring out the specific elements of Catholicism that are objectionable to you and do additional research. There are a LOT of excellent Catholic defense of the many varieties of atheism in quality websites, books, and journals.
• The most common variety of atheism resulted from lack of meaning is because of Nietzsche's pronouncement that God is "dead" (irrelevant) and developed further by Albert Camus who taught that there is no objective meaning in life. The great Catholic apologist G.K. Chesterton's response to Nietzsche can be read here while Camus's error, which in effect is the soul's destruction, has been wonderfully exposed as a logical consequence of existentialism here.
• Many atheists, (such as Edward Feser), have converted into becoming Catholics so they may likely have already considered the reasons for you to become atheist and refuted them.
• Therefore, please BE SAFE and remain a fully practicing Catholic while doing the research. Asking questions is not sin. Having doubts are not sin either, but ACTING on them (by rejecting the Faith) is.

Variations

The answer above is based on a case where all the elements identified above are present in the act. The Catholic Church also considers a variety of cases when only some elements apply. Examples:

• A person who has never been presented with the Gospel (so has never rejected the Gospel), who therefore out of ignorance holds a false idea of God (like Atheism), may still go to heaven. See the Catholic Answer website article on How Can Atheists Go To Heaven?. But in the OP's case, this option is most likely no longer available.
• Heresy, Apostasy, and Schism can only be be committed "after the reception of baptism". Therefore, people without valid baptism cannot commit them. (Most Protestant churches's baptism are valid in the eyes of the Catholic Church.)
• A non-Catholic Christian (like a Protestant) who has true faith in Christ is still technically classified as a material heretic but out of charity, the term heretic is now used only for those who willingly embrace what they know to be contrary to revealed truth (thus guilty in their conscience before God). Therefore, Protestants are no longer automatically culpable heretic.
• Members of the Protestants and Orthodox churches are technically only materially schismatic so they are not culpably schismatic nor are they now called schismatic out of charity.
• A Catholic who out of ignorance holds beliefs contrary to the teaching of the church is a material, not formal (culpable) heretic.
• A Catholic who simply abandons the practice (i.e. no longer going to church) but doesn't intentionally repudiate the faith is not an apostate. But the moment he joins another religion (including falling into unbelief like Atheism), he becomes an apostate.

References used for the answer above

• A 2001 EWTN article published in Catholic Culture website article Heresy, Schism and Apostasy - Definitions by Colin B. Donovan, STL cites the Catechism and the Canon Law during a discussion of the difference of "heresy", "schism", and "apostasy" and divides each into material or formal sin, linking it with the principle of invincible ignorance, culpability, excommunication & other ecclesiastical sanctions, and other related topics.

• A ChurchPOP website article Abandoning Christ: The Forgotten Sins of Heresy, Apostasy, and Schism explaining further what does it mean for those who are guilty of heresy, apostasy, and schism.

• Do Catholics Excommunicate People to Hell? by apologist Dave Armstrong who covers the Biblical basis for excommunication, contrasting it with anathema. He also explains how neither implies condemnation of people to hell but "to warn the sinner of the danger he runs of incurring eternal ruin, unless he repent of his sin", again with Biblical precedent.

• CCC 2087-2089 (within the article about the 1st commandment) distinguishes "voluntary doubt", "involuntary doubt", "incredulity", "heresy", "apostasy", and "schism":

  Faith

  2087 Our moral life has its source in faith in God who reveals his love to us. St. Paul speaks of the "obedience of faith"⁹ as our first obligation. He shows that "ignorance of God" is the principle and explanation of all moral deviations.¹⁰ Our duty toward God is to believe in him and to bear witness to him.

  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.

  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."¹¹

• Catholic Essentials website article on Apostasy quoted a definition from the 1951 Catholic Dictionary, distinguishing 3 kinds: apostasy from the Faith, abandonment of the practice of the Faith, and presumption of Apostasy:

  Apostasy from the Faith is the act by which a baptized person, after possessing the true Christian faith, totally rejects it. The complete abandonment of the practice of the Faith is NOT apostasy, or even presumption of Apostasy. A person is an apostate whether he joins a non-Christian religion, as Judaism, Buddhism, Islam, or falls into unbelief, atheism, materialism, agnosticism, rationalism, indifferentism, or "free-thought". Apostates from the Faith incur excommunication ipso facto, and other penalties"