Since the foundation of the Church, there have been many claims of supposed truth that has come down from heaven after what Jesus, and The Apostles of Jesus have made known in scripture. Three most notable is what founded Islam, Mormonism, and Jehovah's Witnesses. All of these claim an Angel revealed something to their founders, different from the gospel that was proclaimed in scripture; either attempting to correct or add to it. But, interestingly, The Holy Spirit knew this would happen and thus specifically said something about it.
"I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 7 which is really not another; only there are some who are disturbing you and want to distort the gospel of Christ. 8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 9 As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!"
Papyrus 46 is the oldest manuscript of Galatians currently discovered. It is dated around the end of the 2nd century. It contains significant portions of Galatians. With it is Romans, Hebrews, 1 Corinthians, 2 Corinthians, 1 Thessalonians, and possibly others. The reliability of this is unquestionable. It is important to note the date and relation Galatians has with the other scriptures discovered with it. Though the original was written before this manuscript, the date of this specific manuscript is only two of three generations from Jesus himself. Secondly, it is also in agreement with all that which is revealed in the other letters discovered with it. This is important later.
People, at the time of this writing, were 'deserting' the true gospel message preached by the Apostles to a 'different', 'distorted', and 'contrary' gospel. The source of the 'different', 'distorted', and 'contrary' gospels came either from men or even spirits. The Holy Spirit, through Paul, declares that even if the Apostles themselves begin distorting the original gospel preached, they are wrong. Even if an Angel from heaven came and taught a distorted gospel, they are also wrong. In other words, there is no other gospel no matter the source; natural (men) or supernatural (spirits) besides the original gospel that was first proclaimed.
The True Gospel
This is the life, death, and resurrection of Jesus Christ which includes the person and work of Jesus. Jesus, The Son of God and God Himself, willingly, humbly, and voluntarily came down from heaven, to be born of a virgin, lived a perfect sinless life, and was the perfect sacrificial lamb to once and for all atone for the sins of all those who believe. He then rose from the dead three days later validating his power over death and the power as the source of eternal life. This is The Gospel message proclaimed by Jesus and The Apostles. But, as time went on, human nature lead some to teaching something different, distorted, and contrary to this.
Gnosticism
This is a general term given to different early teachings. Some of these early ideas are still apparent in cults and other religions today. There are similar ideas in all these teachings. They deny the foundational aspects of The Gospel message: The sufficiency of scripture, the incarnation of God as The Son and the sufficiency of Jesus death on the cross. Some even deny the death of Jesus on the cross all together. These Gnostics claimed that the source for their teachings came from divine sources and heightened states of spiritual awareness. When people fail to understand and believe Galatians 1:6-9, their hearts and minds are open to just about any claim.
Islam
Around 610AD Mohammed, in a cave on Mt. Hira, hears the an angel who he believes to be Gabriel tell him that Allah is the only true God. Right off the bat, this conflicts with what The Holy Spirit made known 500 years earlier (Galatians 1:8). This belief system is different and contrary to what which was made known by Jesus and the Apostles. It denies that Jesus was the Son of God and God in the flesh. It denies that Jesus even died on the cross. It denies Jesus' sufficient atoning death for sin and denies the sufficiency of scripture. In 746AD John of Damascus wrote about the claims of Islam where he said that Mohammad's ideas of what Christianity believed derived from an Arian monk; of which Arianism is a distortion of the The Gospel as well. Not only would Mohammad's understanding of Christianity be influenced by a distorted gospel to begin with, but he then created his own system from there. Regardless, at the premise, the birth of Islam conflicts with Galatians 1:8. Either Galatians is false, or Muhammad's teaching of a 'different', 'distorted', and 'contrary' gospel is false, both can not be true.
Mormonism
On Sept. 21, 1823 an angel revealed to Joseph Smith of the location of golden tablets and told him that he had been chosen to translate the book of Mormon. The book of Mormon created an entire new system of religious beliefs that splintered off from Christianity. A basic problem of the book of Mormon is that there are no archeological evidences to support any of its extreme historical claims. Mormonism denies that Jesus is the One Triune God, in fact, Mormonism teaches that there is more than one god. They deny the bodily resurrection of Jesus and the sufficiency of his death to forgive all sins once and for all. They also deny the sufficiency of scripture alone and require other divine writings to aid in 'correctly' interpreting scripture. Just like in Muhammad's situation, Either Galatians is false, or Mormonism's teaching of a 'different', 'distorted', and 'contrary' gospel is false, both can not be true.
The Easy Way Out
When an unverifiable person jumps up and declares that he has been given a vision or told by an angel to write a divine writing; it is then easy for them to explain away any true contradictions with scripture. In the case with Galatians, it is then easy to say something like "God isn't talk about me, but others after me". Or, even, claim that Galatians is a corruption of the original. And people have no choice but to accept this unverifiable claim. The major difference between Muhammad, Joseph Smith, and others who make this argument is that God never actually verified their claim for others to see. Muhammad never performed any miracles and his prophecies failed. Joseph Smith never performed any visible verifiable miracles. The prophecies of the Jehovha's Witnesses are continually failed and have been re-explained and failed again. The Apostles were healing the blind and crippled and raising the dead to life. Aside from the biblical recorded accounts, early church teacher, Papias, validated this as he knew of people who were raised from the dead in his day. Membership growth and wining Wars do not validate someone's spiritual truth claim. Atheist church membership is growing and atheists have caused and won many wars. Declaring "I will raise myself from the dead three days after I die" and then doing just that is validation. Declaring that Jesus is the way, the truth, and the life, then raising people from the dead is validating the source of power. Anyone can claim that Galatians is wrong. A 12 year old can just make that sort of claim just to reject the authority and justify themselves, but again, that does not make is so. Its easy to make any claim, deny facts, and avoid being verifiable.
Not a Forgery
So how do we know that Galatians is not a later forgery with the name of Paul as the author later inserted? Well, its quite simple actually. We just compare the vocabulary, writing style, theological point of view, cultural first hand knowledge, and presupposed historical situation to all the other Pauline writings. Clement of Rome (and other early ancient church teachers) quoted from Romans and he wrote around 90 AD. So we have outside sources to validate Paul's Letter to the Romans. When comparing such validated letters we see the commonalities of the letters from one author style. Such is with Galatians, it is the same in vocabulary, writing style, theological point of view, and presupposed historical situation as the other validated Pauline writings. Now of course no one now saw Paul pen the letters first hand; just like all other ancient writings, yet, given the evidences, we can be certain that this letter is in fact from Paul. Even in Islam, Muhammad never wrote anything; Joseph Smith never showed the supposed Gold Tablets to anyone; Jehovah's Witnesses only have the unverifiable claims (and failed prophecies) of their founders.
This is a major problem for Islam, Mormons, and Jehovah's Witnesses. They are forced to claim that the words in Galatians are not of Paul but must be later insertions and corruptions. They are forced to go against the certainty of evidence support for the authenticity of Galatians. Because if they admit the truth of Galatians, they are then exposed to the truth stated by Paul in Galatians 1:6-9. So, of course, they take the easy way out and just flat out deny it.
Conclusion
Sense we can be certain that Paul, an Apostle of Jesus Christ validated by the other Apostles (Galatians 1:18-19, 2:9; Acts 9:26-29), wrote Galatians and a copy exists from the 2nd century we can conclude that his statement in Galatians 1:6-9 is from an Apostle of Jesus Christ validated by the other Apostles. Therefore, the Apostolic statement must be true. That if anyone and even an angel preaches a different gospel message, it is wrong. Islam, Mormons, and Jehovah's Witnesses do just that, preach a different gospel from other angels. But unlike them, he is validated by Apostolic support and miracles through preaching the true gospel and declaring Jesus as Christ and eternal Son of God. Sense Galatians 1:6-9 is true, the claims of Islam, Mormons, and Jehovah's Witnesses can not be.