Christianity, as a whole, only believes in one Gospel.....that which is contained in the Holy Bible.... and "only" in the Holy Bible. Yes, there are different denominations, but essentially those differences are in worship style (open or charismatic, conservative, etc.). Each of them still only follow the precepts and instruction from that one Holy Bible.
There is a scripture in the Bible which alludes to other practices:
So it seems to me that you're saying that General Christianity believes basically the same thing but each denominations choose to teach it or embrace it differently. I'm not sure how that jives with some churches requiring infant baptism and others not or even differing in the mode of baptism (sprinkling or immersion). Or another example would be sacrament or communion. Don't Catholics teach that the Holy Communion or Eucharist transmutes into the body and blood of Christ? Maybe I'm splitting hairs on that one since we believe it to be a representation of the body and blood of Christ, but I think the principle is the same. These groups all take there doctrine from the Bible but have interpreted it in such a manner that their practices, although consistent with the need for baptism or some form of bread and wine sacrament, differ in how it's implemented.
To what Gospel are you referring to when you say that there's one Gospel? Are you referring to the Bible itself being the only Gospel? Or are you defining it as the doctrine taught by the prophets and Christ contained in the Bible? To me there's a fundamental difference. If you're referring to the Bible itself then it seems like you're placing a lot of stock in a bunch of pages bound together. If you're talking about the contents of the book, the actual teachings/doctrines found within the book, then I completely agree with you. There is only one Gospel. God isn't going to have a set of instructions for some people and not for others. This is again where people can choose to disagree as to what those instructions are.
Everything the LDS church teaches has its basis in the Bible. The LDS church believes that God hasn't stopped communicating to us through prophets. If he did it in the Old Testament and even after Christ's resurrection and ascension into heaven, why would he stop now? Because we now have the Bible? Why would those people get a prophet and we don't? Because we don't need one? That would be pretty short sighted if the answer to that question was yes. Christ felt there was a need for apostles and prophets even when he was on the Earth, but not now? Joseph Smith didn't do anything different than the prophets and apostles of old -- he taught from the scripture that was already available (both the Bible and the Book of Mormon), and corrected incorrect doctrine just as Paul was doing in all his epistles. If Paul was having to do it just a few years after Christ ascended, it seems that there's a good chance that a few things might have gotten distorted in the 1800 years between Paul and Joseph Smith. Then again, if ones faith is based on a book instead of the doctrines found within that book, then I can see why that person would think there was no need for correction. If I tear a page out of the Bible am I now missing a part of the Gospel? No. Of course not, because the Gospel is bigger and greater than the Bible itself. I'm sorry if that comes across as crass. I believe the Bible is a sacred book and I treat mine with respect. I'm just pointing out that the book itself isn't the Gospel; what the prophets taught and is recorded within the Bible is the Gospel. Which leads to my confusion as to why any religious person would scoff at the idea that a prophet exists and is in fact needed when there's clearly disagreements as to what the Bible or Koran or Torah or any other religious text is actually teaching.
Obviously I'm speaking mostly from a logical standpoint, mostly because a lot of detractors think Mormonism is illogical, but if you'd like, I can site Biblical scriptures for you that support my argument. But that just leads into the problem Maris eluded to about interpretations of Biblical verses. Of course, if there were a prophet to interpret and instruct then there wouldn't be much of a problem, but I digress...
I think Maris' comment about different Bibles means that whatever version or edition of the Bible you're using differs in wording from the King James Bible I use -- Galatians 1:6-9:
(6) I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: (7) Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. (8) But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. (9) As we said before, so say I now again, If any man preach any other gospel unto you that that ye have received, let him be accursed.
I fail to see how this scripture is an illustration that the LDS church teaches a different gospel than the one Paul says is the true gospel. Paul is talking to people who have been recently converted to the Gospel and warning them about simply accepting what someone says is the Gospel. Read a little further:
(11) But I certify you, brethren, that the gospel which was preached of me is not after man. (12) For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ.
He's basically telling the Galatians that everything he knows and has taught was gained through revelation. As a recent convert himself, he obviously didn't study the Old Testament prophecies and teachings in a Synagogue. Nor did he appear to have texts from which to learn Christ's teachings. This chapter is more about seeking personal revelation to know the truth than it is about chastising and warning people that there are going to be those that will teach incorrect doctrines.
Again, I have no problem with people believing whatever they want, so hopefully this doesn't come across as though I'm attacking anyone or a belief system. I'm just trying to explain what
I believe to be misunderstandings and false statements.