Although I am a staunch Catholic, I might have to disagree with the view that:
the only religion that contains the entirety of truth as it has been revealed to humanity, is Catholic Christianity.
I'm not sure that the "Catholic" denomination/church is the only "Christian" denomination/church with a dogma/belief, and especially an "understanding", of the "entire truth" of Yahweh, as it has been revealed to humanity. Although I do believe that non-Christian religions do not/have not been shown the entire truth of Yawhew, I do believe at least some non-Catholic, yet Christian, denominations/churches have, through spiritual means, been shown/learned, much of the truth. I doubt that most Christians have learned/understood the entire truth, and I suspect that not even the "Catholic" church has learned/understood it all.
There are far too many interpretations of the "truth", as we define it, in the Bible; even if the entire "truth" is in the Bible. The problem arises when we individually or as a group (as in "Catholics" or "Lutherans" or whatever) try to set down in stone exactly what it all means.
I believe there is validitiy to many different beliefs/interpretations in many different Christian denominations.
I don't mean this as deragotory, but once we, as Christians, stop looking at ourselves as Catholics or Baptists or Lutherans, etc., and look at ourselves as "Christians" and part of the Christian religion, belief, church, FAMILY, we will get along better, and our collective beliefs may/will hopefully lead us to the "entire" truth.
In my opinion we should be Christians first, and Catholics second. It is after all the "Christian" religion.
I suspect that because the beliefs of the different religions vary so much, that they can't all be right. I also suspect, that IF there is a divine, loving and all-powerful "God", He will show us HIS truth.
I guess it all boils down to the question: If not all religions agree (which they don't), which one is the right and "true" religion? Christians, which make up about half of the world population, believe Christianity is the "true" religion.
As to whether or not there should or will be one religion; IF God is real, and WHEN Jesus returns to earth as it states in Revelations, THEN (and probably only then) I can guarantee you there will be only one religion, Christianity; and there won't be a number of different denominations, there will be just one the "Church of Yahweh".
May God bless all of you, your families, and your Christian outreach to others,
T.R.