Monotheism is the canon that there is only one god. You see, before monotheism was "in", people had differnet gods for different things, a god for good health, a god for battle and so on. And whichever it was they needed help with at that moment, that's the god they would pray to. But monotheism is very strict on this, only one go-to-guy.
If you consider the implications, they would have to be wide ranging. First of all, from an administration point of view, it's a lot easier to send all your mail to the same guy, regardless of your case. Secondly, you don't have to worry that some gods would feel slighted because you constantly do business with others.
Since there is only one god, however many "religions" you could invent, they would always address the same guy. And indeed, this is something the major religions accept, that through Judaism, Christianity and Islam, it is the same god you worship. This is a very troubling truth, for several reasons.
If there are several paths to god, several ways to reach him, which is the best one? Which is the way that given a lifetime of deeds based on those "guidelines" gives you the highest "score" with god? What is the best way to ace the test? Is god going to reward followers of one religion higher than followers of another? That doesn't seem fair, does it?
I think intuitively we tend to believe that god *is* fair. It *does* pay off to be a good person, god will reward those who are good and punish those who are bad. Isn't that what we believe? So he shouldn't treat you better for picking one religion over another, should he? Because they are all ways of reaching him.
But if all [monotheistic] religions are equally good, it means that they are instances of one another, that they are redundant. Consider the following illustration from mathematics.
f(x, y) = x * 2 + y * 2
g(x, y) = (x + y) * 2
f
and g
are functions. In fact, they are equal, because they produce the same output for the same input. As long as this condition holds, it doesn't matter what happens inside, it doesn't matter how the output is computed. All that matters is what the result it. It may be that g
is more "clever" in computing the result than f
, but both do the job just as well. What it boils down to is that you only need one of f
and g
.
This is a mathematical illustration of what we've already established about religions. All are ways of reaching god, equally good ways. However it is you go about reaching god through these different paths, ultimately gives you the same outcome. And that means.. one is as good as the other. There may be one that is more "clever" or "efficient" or "easier", than another. And if so, why wouldn't you pick the most "clever" way, just like a mathematician would use the function that gives him less work?
If two religions don't specify that you must be a lifelong follower to be worthy of god, we can consider them interchangeable. In Christianity, you can become a follower at any point in your life, and god won't reject you. If that's also the case with Islam, then you can switch between them.
In particular, this gives you flexibility. If as a Christian you move to a country where Islam is the standard, you can switch and not "miss a step". Or if you don't like praying as much as you should in Islam, you can become Christian and pray less.