The answers given in this article are based on Catholic teaching. The longer and much more detailed theological explanation is in my book: Forgiveness and Salvation for Everyone. This article summarizes the conclusions of that book.
Everyone who loves others, who truly selflessly sincerely loves other persons, must have the infused theological virtue of love. No one can love others in this transcendent full manner without that virtue, which is a gift from God to the human person. And everyone who does truly love others and who dies in that state of loving others, will have eternal life in Heaven.
The reason for this is that love is a virtue which can only be possessed as a gift from God along with the state of grace. The virtue of love and the state of grace always includes also the virtues of faith and hope. A person can have faith, without love and hope, but everyone who has love, has the other two infused virtues, faith and hope. And this “state of grace” is therefore essentially the state of loving God and neighbor, just as the Gospel teaches, we must love God above all else and love our neighbor as ourselves (Mt 22:37-39).
Christians receive the virtues of love, faith, and hope, and the state of grace in the Sacrament of Baptism, called baptism by water. But non-Christians can also receive these same gifts from God by a baptism of desire. The baptism of desire can be explicit, as in the simple case of a person preparing for baptism, who desires to receive it. But the baptism of desire can also be received implicitly, as when a non-Christian seeks to selflessly love others, and this desire for a selfless love is a desire for that virtue of infused charity which can only be received from God in the state of grace along with faith and hope.
Sometimes this love is a love of God and of neighbor, as when a believer in another religion loves God, according to their own religious beliefs and also loves other persons. But if the person has religious beliefs which do not include belief in a loving God, they can still receive this implicit baptism of desire and the state of grace by seeking a true love of other persons. God then gifts the person with the virtue of love.
And this implicit baptism of desire can also be received by persons who do not believe in God at all, such as atheists or agnostics. For in loving their neighbor, they implicitly love God. All that is good and worthy of being loved in one’s neighbor is from God. Everyone who loves others necessarily always implicitly loves God, even if they do not believe in God. And this love is accompanied by faith and hope and the state of grace.
How can an atheist or agnostic have faith, when they do not have a belief system in which to place their faith? By loving others, they implicitly have faith in all the transcendent goods of this life, which are from God, such as love, truth, justice, mercy, and so on. By loving others, a human person implicitly has hope for all that is good for other persons and for oneself. In this way, even one who does not believe in God can have love, faith, and hope, and therefore can have the state of grace.
Are all human persons saved? No. There is a Hell, and very many human souls are sent to Hell. God wills that everyone be saved; the offer of salvation is universal. But some persons reject the love of God. They do so by an act of actual mortal sin, which is a gravely sinful act, chosen with full knowledge and full deliberation. This type of sin is a full rejection of love, and results in the loss of the infused virtues of love and hope. Sometimes faith remains, without love and hope. Other times all three virtues are lost. These persons must repent and be forgiven in order to return to the state of grace and the path of salvation.
What happens if a person who does not believe in God, and who is in the state of grace by this implicit baptism of desire, falls away from the state of grace by an actual mortal sin? In that case, the person can return to the state of grace by the same selfless love of others, which love constitutes an act of implicit perfect contrition. Since perfect contrition is sorrow for sin out of love for God, this same selfless love of God implies a rejection of all that is gravely contrary to love and a repentance from the same, including one’s own past mortal sins.
Can unbaptized infants be saved? Yes. See this article.
Who can be saved? Everyone who selflessly loves others, including Catholics, non-Catholic Christians, Jews, Muslims, other believers, agnostics, atheists, and persons of every other belief system, as long as they love others.
What if someone loves others, but commits an actual mortal sin and does not repent? That is not possible, as an actual mortal sin by definition is contrary to the virtue of love and results in the loss of that virtue. In addition, anyone who does sin in that way returns to the state of grace by an act of selfless full love of God or neighbor.
What if a person is a Satanist or a terrorist or a Nazi or the like? Anyone who worships evil, or who is filled with hatred for other persons, therefore does not have selfless love of others and is not saved.
How many persons go to Hell? Very many. There are many persons in this world who are entirely selfish, and who only seem to love others in so far as those other persons fit into their own expectations and needs. Then there are many other persons who have decided to hate others. You cannot hate others, and love selflessly by that gift of God called the state of grace.
Heaven is a place of eternal love, and so the path is like the destination: love.
Ronald L. Conte Jr.