The common sense view of how we get to know things is that there are things out there in the world which we can sense, and discover things about, and then somehow make linguistic pictures of those things which we then pass around to each other so that language is a mirror of nature and of reality. Propositions correspond to the way things are in the world- if they correspond, they are called "true" and if they do not correspond to "reality" they are termed "false".
This is called the "correspondence theory of truth" and is extremely simple to understand- if the statement corresponds to reality, the statement is true. This is the way the Greeks saw the world, and the way we have traditionally understood things as the "man in the street" understands them.
But there arises a problem. Do we actually see things as they "really are" or is what we see a construction put together by our brains?
We know that light enters our eyes, for example, and that light is something like a wave and also something like a particle, which in some sense "vibrates" at a certain frequency and is reflected into our retinas, causing specific nerve cells to respond to these vibrations and then firing, and stimulating the optic nerve, which then transmits the signal to the visual cortex of our brains, and then because our brains are constructed as they are, we then experience what we in language abstract to be a "color" and we again abstract that impulse, connecting to verbal centers and we call or define that particular stimulus with the word "red".
But is that object "actually" red, or is it just reflecting light of a certain wavelength which our brain defines with the word we have learned to use which is associated with that brain state? In other words, is that thing "out there" actually "red" or is it just a label our brain affixes to the EXPERIENCE of what is called (defined to be) "red"?
And what is the relationship between that experience called "red" and the word "red"? Do each of us experience "red" the same or not? Color blind people do not experience certain colors the way the rest of us do- we know that because sometimes they cannot tell the difference between different colors. That is the only way we surmise they see things differently than we do because we have an experience, which perhaps we call "aquamarine" which they apparently do not have, since they cannot perhaps distinguish it from what we call "blue" and what we call "green"
So what is reality? Is it what is out there or is it what we experience as out there?
This problem has been one discussed by philosophers for the last two thousand years. Perhaps you have seen it in the form of "if a tree falls in the forest does it make a "sound"? It is essentially the same problem. We presume that sounds are vibrations in the air which our ears sense, and we call that process "hearing". But the problem becomes, if a tree falls and creates such vibrations in the air, should such vibrations be defined by the word "sound"- a word which connotes that someone actually sensed those vibrations and "heard" them- or is such a use of the word inappropriate in such a case?
Well it doesn't take too long before one ends up saying "Who cares? What difference does it make if we call it a "sound" or not? Did the tree really "fall" in a humanly understood sense, and did it make a "sound" in a humanly understood sense. Was that even "real"?
But honestly, who cares if we call such an event as "real" or not?
Isn't that about the way you are feeling right now?
At this instant, if you are feeling that way, you are what we call an "Epistemological Pragmatist" which is a fancy way of saying that you understand that we are now quibbling about definitions and in the geneneral course of things, it really doesn't matter what one calls "real" anyway. Is it "true" that the falling tree made a sound or not? After all, it must be one or the other, right?
Not really. It is a matter of definitions. If you want to interpret the question one way, it is "true" or you can interpret it another way and it is "false".
In cases like these, truth or falsity doesn't really matter much- it just depends on how we somewhat arbitrarily decide to look at it.
I am sure you have seen pictures which you can look at and see either as, say, a rabbit or a duck, or an old lady or a young lady or a staircase going up or going down. Such pictures are designed specifically to be perceived in an ambiguous way, and it all depends on your interpretation of what you see.
These are some of the central problems with the correspondence theory of truth. What is "real" in this way of seeing things? Does it matter how we define what is "real"? How can propositions "correspond" to what is "real" if we cannot define what reality is, or what that correspondence could possibly mean if it is all a matter of our perceptions and how we define them?
These are the issues philosophers have grappled with.
The Pragmatist is one who sees all of the world differently than those who see true statements as "corresponding to reality", so they effectively avoid all the issues such a viewpoint raises. The Pragmatist sees these issues pragmatically- true propositions do not "correspond" to a "reality" beyond what we can see feel and touch with our senses or our emotions- for all practical purposes there is no separation between what is real and what we experience- experience IS reality.
Since experience IS reality- there is, practically speaking, no difference between appearance and reality.
Truth becomes a property of propositions and linguistic statements- it is not a property of reality because we cannot "get down" to a reality beyond our experiences of it. We know how to use the word "true" and "false" and those meanings depend on linguistic contexts- not on any state of some un-perceivable "reality" which only exists in our imaginations.
Truth is always based on some practical use for a particular proposition. "The car is red" is not about a state of the world, but is about our linguistic statement of some experience we have. The car is red because we perceive it as what we call, or define as the color "red".
The practical use of such a statement might be to distinguish a red car from a green one etc.
The practical use of a scientific description would be the fact that such a statement works better in achieving a practical result than some other statement. (Copernicus works better than Ptolemy, Einstein works better than Newton)
Such scientific understandings change from time to time in paradigm shifts which change our definitions and understandings of our collective experience. Copernicus did not change the observational "data" just its interpretation.
Similarly, moral propositions are "true" because they work better socially- societies which do not sanction murder for example are more desirable to live in that those who might oppose those ideas.
So what does this have to do with religion?
Like all other propositions, religious beliefs have nothing to do with some abstract "reality" we cannot see or feel- they are judged by how they function in our lives in giving our lives meaning and peace.
Statements about God, like the statements about the "red car" are about out perceptions and experiences, not about anything else. Religious statements are subjective in the sense that the difference they make is within us- in personal experience, and are not verifiable by others.
But all of the most important decisions or ideas we have as human beings are subjective as well- we decide whom to marry, what is right or wrong, whether or not we should do something perceived as "wrong" or not, based on subjective feelings. There is no objective scientific way to decide what is right or wrong in our lives- whether or not abortion is wrong, which political party to vote for, which church to join, whether or not the historic person Jesus was the Son of God and our redeemer, or not.
In fact, the very question of whether or not we are free or determined can be seen to be based on the difference between subjective and objective propositions, but that discussion is for another post.