I am an atheist now, but I was raised as a Catholic and remained one until twenty-one. I was taught to look at Protestants as unfortunate, having accepted Jesus as their savior, but not the Pope as his representative on Earth, and consider them as similar to the Prodigal Son in the similarly-named parable. Orthodox religions were almost okay--they just needed to come back into the fold and accept the Pope as the leader of Christians on Earth. Jews were God's Chosen People, but since they did not believe in Jesus as the Messiah, they were unfortunately going to Hell--unless they were converted. All in all, Jews weren't bad people either, because at least they believed in the same God as us, and were part-way there. More distant religions like Islam, Hinduism, and Buddhism were given little or no examination, but more disparate branches of Christianity (in Catholics' mind) like Mormons, JW's, and Seventh-Day Adventists were considered cults because they were believers in Christ but with dangerously heretical beliefs mixed in that no self-respecting Catholic would ever accept.
Of course, when I started actually examining all of these religions on my own and discovering for myself my own opinion on them, I began to see things much differently.
Religion, in itself, is simply a belief system one person has claimed as the truth, and then other people come to follow it. In actuality, no two humans believe exactly the same on all topics, and so, to be honest, we could say that all human beings have their own religion, as varied and different as fingerprints. It is funny how Protestant religions operate on the doctrine of a personal relationship with Jesus, but don't take it all the way under that line of thinking, and consider that following any other person's religion would mean you weren't truly in a personal relationship with Jesus--you were copying another person's personal relationship with Jesus as your own.
It's all immaterial to me now. After exposure to so much information of all the world's religions, I developed the idea that not all of them could be right, since so many were diametrically opposed or made claims that ruled out their both being right...and I made that common leap of logic that came next: If many of them must be wrong...could all of them be wrong?
I realized that most people don't research and chose their religion--they are indoctrinated to it as an impressionable child by their parents. Most people simply fall into their religion and continue it out of familiarity. It is an accident of birth for most people on what religion they espouse. With this indoctrination comes rigidity and resistance to other ways of thinking, thus the all-too-common hostility to other religions...though it is telling that most religious hatreds are between more similar religions than those very different. You don't hear about Christians hating on Hindus for having multiple gods, but look no further than Northern Ireland for Christians behaving in very un-Christian-like ways towards one another. Likewise, Jews and Muslims can profess belief in the same God, Abraham, and not eating pork, but that pesky part in the Bible about Abraham's sons and who got his blessing kinda tears the agreements in two after that.
No religion has the authority to declare if another religion is legitimate or not, as none of them are based in any way on rationality or facts, but faith--the very antithesis of fact--believing in something with no evidence whatsoever. When new religions pop up, or old religions die away, contemporary religions have no problem attacking their legitimacy on grounds of too few adherents...but they forget the fact that, at one time, their own religion was a foundling with only a handful of faithful.