We have law schools which graduate law students schooled in pretty much the same way from one to the other. We have engineering schools which teach their engineering students in pretty much the same way. We have many Christian Bible colleges which graduate our pastors, evangelists, and clergy supposedly using one basic book (the Bible) from which to teach all these graduates. Why then since basically only one book is used to teach so many are so many taught totally different things?
A sinner who has never been brought up in a church is surely at a disadvantage. He turns on his television and here is a preacher teaching that he is hell bound for his sins. He turns the channel and here is a preacher teaching all are saved and there is no hell awaiting him. He later turns the channel and here is a preacher teaching that the church will be zapped away in a pre-trib rapture to safety while he will suffer unimaginable horrors and everlasting punishment. He turns the channel again and he finds another preacher preaching that there will be no rapture at all of the church or anyone else. He turns the channel yet again and here's a preacher speaking in some kind of gibberish tongue. He throws up his hands and declares that all those who spent their time and money in all the different Bible colleges should get an immediate refund.
The core dogma of the Bible and of Christian teaching is not all that much different. When we start judging you or others, that is where a lot of the trouble lies.
The reason there is so much differences amongst denominations is because it is all being preached by sinners. No one is given crystal clear certainties on all supporting teachings. But if anyone says Jesus Christ is not the Son of God and Savior of mankind, they are not Christian. If they deny the existence of heaven or hell and a judgment upon death, they are not Christian. If they tell you what will happen to you for certainty if you do not do X or Y or if you do do Z, they are not Christian either, at least, they are doing God a great disservice.
FYI, there are 10,000 protestant denominations that do not agree with each other many supporting doctrines or teachings, such as the rapture, and what it takes to be saved, etc. There is only one Catholic Church that has, for all intents and purposes, remained steadfast in its dogmas and doctrines for 2000 years. Think on that. All those others emerged 1,500 years or more since Jesus resurrection.
But I do not care to argue with any of them. I will say this and they can say what they want --- all the great miracles that occurred in front of throngs of witnesses, all the well documented apparitions of the Virgin Mary, all the stigmatas on saints bearing the wounds of Christ, all the weeping statues that science cannot explain --- all these manifestations of this nature occurred to Catholics. If God is speaking in ways other than His Word, it may be through His signs and wonders for which I, for one, am eternally grateful.