In the discussion of this subject, we ought to remember three ancient distinctions in philosophy:
Objective-Subjective
Formal-Material
Actuality-Potency
Objectivity is as a thing is in itself. Subjectivity is as a thing is perceived by a rational being. Objectively, I am a human male; subjectively, I may believe I am a reptilian female. It doesn’t make me a reptilian female, really, but I may be excused for thinking it.
Formal recognition is direct, conscious, and assented. Material recognition is indirect, subconscious, and not assented. Formally, I may be convinced when I say “I am a female reptile”; materially, I am absolutely wrong. My mind will be in accord with reality when someone gets me to formally recognize what is so far only true materially. That person is very merciful to me if he insists that I am not a female reptile.
Actuality is the state of a thing being truly real; potency is the state of a thing being possible. Actually, I am not Pope; potentially, I can say “I am Pope”.
Within Christianity, this system of using good philosophical distinctions is very specific to the rational thought of Catholicism. It is helpful here.
A final crucial thing to know is the Law of Non-Contradiction. It is a basic philosophical precept that is common to all clear thinking. For example, something cannot Be and also Not Be at the same time. For another example, if “A” is exclusive of B, “A” can never be “A+B”.
We believe Christianity is the only true religion because Jesus Christ said so. We can see from the* undeniable historical fact* of His Resurrection that His claims were true.
These are the chains that link the Truth of our Faith together. If Jesus isn’t raised from the dead, Christianity is meaningless; if Jesus is raised from the dead, everything He said takes on absolute significance for all human beings. If Christianity (A) is true, by a process of logical exclusion it must be the sole true religion simply because all the other religions (B) contradict it in some way. The Law of Non-Contradiction holds here, as anywhere else. 2+2=4, always. To say Christ co-exists with false religions is to say 2+2=5.
Now, to apply these categories to your question:
What happens to someone who never had a chance to hear about Jesus? Objectively, as a rule, “anyone who does not know Christ does not have life in Him”; subjectively, the** individual** pagans, atheists, or whoever else, cannot possibly have known Christ. He judges them based on what system of virtue they* did* know, as subjects (persons).
Formally, they never knew Him as a person; materially, if they grasped some truths about life, death, virtue, sin, good, and evil, they knew Him in a shadowy way. After all, He says: “I am the Truth”, so any Truth in this world is a small participation in Him.
In actuality, Christ only saves those who live in Him by faith & love; in potency, Christ saves all men regardless of who they are or what they believe. Note well the distinctions. Christ saved all men potentially; He saves Christians actually, but exceptions are made because God is a living being, not a slot machine or computer that produces results based on exact numbers. He chooses, decides, and - most importantly - loves.
Now do you believe something Wet can also be Dry simultaneously? Of course not. It is a contradiction. So, when Christianity affirms, with the Bible, that the “gods” of the Gentiles are “Demons” (Psalm 96:5), and there is only one true God, what do we make of that? Either Hinduism’s many gods are true, or else this is true. We cannot be dry and wet at the same time. This is about basic truth. Believe it or not, like it or not, it is what the religion claims and it is logical.
We leave all things up to His Mercy. This really is a tiresome topic to debate.