Well, I wandered into this thread. It reminds me of the same debates about the Mormon church I heard growing up in Salt Lake, in high school and college. Neither I nor my family have ever been LDS, but we grew up with it all around us.
I may not have much to contribute to this conversation other than to say that there appear to be some good people in bad religions (or no religions) and some bad people in good religions as well. What ought to matter most is the quality of a person's thoughts and actions, the kindness, honesty and fortitude with which a person lives his or her life. There are some basic principles that help people live a good life, and morality has a very real effect not only on a person's life decisions, but also on that person's health and whether that person can bring benefit to others. Whether moral and inspired teaching takes the form of doctrine or not may be a matter of formality. That is to say, a person might memorize scriptures and participate in formal ceremony, but if he/she is not sincere and goes on to act as usual the other six days of the week, anyone with intelligence (including God, presumably) might view that person's religious behavior as meaningless.
Historically speaking, formal religions have presented themselves as the single, true path because perhaps they were--at least for their respective followers. If one is to believe that Man was made in the image of God and then observes the various appearances, cultural forms, languages and behaviors of the people in our world, one will quickly notice that we are not all the same. Different peoples might have varying characteristics. The principles that they enlighten to might be different. The way that lives exist and the way that energy matter circulates in their systems as celestial beings may be different. So if a person were to embark on a journey of spiritual ascension and mix their path with another, their success might be undermined. This is one of the reasons why religions have put down other religions. The idea is to have people commit to a single path and not mix things up.
Of course, what I just described is based on the belief that man can ascend beyond this mortal world in which we live. If the purpose of a person's religion is simply to help that person find comfort, be a good person and live a more pleasant life day to day, then keeping spiritual paths separate would be less critical.
As for whether a particular belief system or church is authentic and able to have its believers truly improve and elevate themselves, that is something that each person needs to determine for him or herself, relying on their own powers of perception and discernment.