I think my response to you also responds to the others who quoted me.
I'm going to try to simplify theological and ontological concepts which you do not agree with the truth of
For someone who believes in a loving, just but also merciful God, it is God who decides who is "saved" to use that term. Not us. Now, people through their choices may ultimately exclude themselves from salvation, but that is an exercise of free will, not God being deliberately cruel. God wants all people to share in salvation but also respects the choices humans make. One reason Catholic teaching includes purgatory, which many other Christians find laughable, is that it's a serious attempt to reconcile an infinitely loving God with the existence of hell. Free will is part of it (someone choosing to do what they know is evil, even if they don't believe in God, is still knowingly choosing to do evil, and is culpable for that, even if not for their unbelief). The other way round, the Catholic church that some specific people are in heaven, but it doesn't deny others are, nor does it teach that any specific person is in hell. I think there's also a theory of some that hell exists, but is empty.
So re unbaptized infants, or simple unbelieving atheism (not rejection of a God one believes in!) it is a matter of entrusting their souls to a loving God (limbo was a theological theory, not dogma). Or a story from earlier this year, something like Pope Francis told a young boy that his dead father may be in heaven. He didn't say he was, he didn't say he wasn't, because it's God who decides.
Theodicy (reconciling the existence of evil and suffering with a loving God) is a separate thread, perhaps...
Only some aspects of faith are considered inerrant - popes are just as fallible as any person generally, including on moral teachings. It is taught/believed that the Cardinals in selecting a new pope are guided by the holy spirit, but the former Pope Benedict noted (when he was Cardinal Ratzinger I think) that just from looking at the behaviour of some historical popes, sometimes the guidance of God was ignored in choosing a new pope (I'm paraphrasing from memory).
Vatican II reaffirmed the primacy of the individual conscience (contrary to the notion that error has no rights). To oversimplify, one is morally obliged to follow one's conscience, even if it runs contrary to what the church says. It must be an informed conscience, one should not disagree lightly, but even if you end up excommunicated you're obliged to follow your conscience.
Also, sin is most severe (aka mortal) when it is made with full knowledge and appreciation of the consequences. An atheist by definition is incapable of such serious sin in terms of denying God, because they don't believe in God. Like I said before, it's not a choice. Someone can only knowingly reject God if they believe in God and understand the consequences. Which is inapplicable to an atheist. Because even if they understand what Christians (or for other religions, those religions) say, they don't believe it.
So someone who was angry and upset with God could reject God in a very serious way aware of the consequences, but not someone who didn't believe in God.
This presumes no wilful blindness, e.g. I can think of one atheist I knew who felt called from their secular university studies of comparative religion or something like that and ultimately became Catholic (which surprised their atheist friends!). If they had felt called and deliberately ignored that, they would have been more culpable because then it would have been rejection, not lack of belief.