If there can be only one ultimate truth pertaining to anything, then only one of the world's religions can have the ultimate truth. One test would be in God demonstrating and proving to the world who he is, which would bring us first to Judaism and then to Christianity, after God more fully revealed himself along with further proof. And if a particular Christian faith is the One True Faith, then it would have to be the one that Christ himself established, not some variant that a mortal man invented 1500 years or so later.
Did you notice how many "ifs" you employed? That in itself suggests quite a bit of uncertainty.
But the fact is, no matter how you try to slice it, the God we find in the OT is truly not the God we find in the NT -- therefore the Jewish and Christians Gods are not one and the same.
But you ignore the fact that this "one God" is supposed to have revealed Himself to Adam, and then to Noah, and then to Abraham, and then to Moses, and then to Joshua, and on and on and endlessly on through prophets that all saw this "one God" completely differently. And in every case it is -- if you do the careful work of reading -- a different God. It is, in every case, a "god" that reflects the notions of the writer. Doesn't that tell you anything? Really?
And then I have to ask why 1500 years later? Why not 500 years (that would be Mohammed)? Why not 1800 years (Joseph Smith? This is and has always been one of my most important questions about God, and nobody ever -- EVER -- answers it. If God is truly God, and can reach us all and is all-knowing, then this God MUST KNOW that the use of such prophets is futile and must go wrong. And we know today that this is true, because it DOES god wrong.
So we are told to beware "false prophets," and given some silly notions about how we can tell which prophets are false and which aren't.
But think very carefully about this for just a moment -- if you can tell, out of your own apprehension, which prophets are true and which false, then that can only be because you have received the wisdom of God. And if everybody can do that, then everybody has received that wisdom -- and all of us are prophets.
Now, what does that tell you about "truth?"