These 25,000 denominations are claiming some absolute truth and they collide, well ... that is better than saying there is no truth, or saying we can not know truth or saying what is true for you may not be true for me so..... why do not we just sit down around a camp fire and sing "Kumbaya my Lord"? Or my RC friends might say, yes there is truth and the Pope says whatever it is, so why don't we just gather around that (please,... I say this respectfully and with no sarcasm)? So what should we do, should we go to the post-modern solution of relative truths?
It is interesting that when Jesus prayed for unity he did not leave out the notion of truth, in fact several times this word were mentioned.
Sanctify them[b] in the truth; your word is truth.18 As you sent me into the world, so I have sent them into the world.19 And for their sake I consecrate myself,[c] that they also may be sanctified[d] in truth.
I contend that the only reliable place to look are the Scriptures because Jesus gave authority to it, not the Church. It was better than that, it was the head of the Church who said that - Christ the Lord.
There is an empirical test to this. See if what Scripture says about your sin matches your experience, see too if you can by your own efforts live out the demands of God. My answer to the first is "yes", my answer to the second is "no" and the solution is not to try harder.
Lastly in the creeds we confess "we believe in one holy catholic (universal) apostolic church... we believe in the communion of the saints". This is an article of faith, that it is , it is already true now, that there is one church -- a body of believers (ie not hypocrites) who in repentance and faith cleave alone to the work of Christ for their sins.
I do not see this oneness by my eyes, that is why it is an article of faith the same way I have not seen Jesus died on the Cross for me, but I believe that story is true.