Actually, this is the very reason I'm in university, to bring about that change - I have some very good ideas which, if properly implemented, will bring sanity back into civilization, bringing all religions into healthy and balanced alignment (they're all saying the same thing, using different "terms of the art", bring religion, science, politics and even political parties - within and between every nation - into a balanced and proper alignment (yes, this is possible - when correctly understood and implemented they don't conflict with each other).
I'm there (at university) to develop the skills to communicate and implement these ideas in a clear and unambiguous manner.
In fact, I would like to form a school for this very purpose - but I'm no (sane) George Soros who can afford to just start building.
A very dangerous belief!
They most certainly are not all saying the same thing. In fact, most of them are mutually exclusive by design. The Living God of the Judeo-Christian religion says that Jesus is the only Way to God. John 14:6, "I am the way and the truth and the life. No one comes to the Father except through me." Islam is very clear that Christianity is "amiss" according to them and is entirely predicated on Mohammed's opinions basically and an unwitnessed event/occurrence. Mormonism is founded on the beliefs and unwitnessed experiences of Joseph Smith. Buddism, Hinduism, et al. are both broader in origin and had more inputs than a single person.
The list goes on, but by virtue of their beliefs, they are necessarily mutually exclusive most of them. Regardless, Islam calls Christianity and Judaism completely false. Ergo, how can the two "both be right." They can't.
The exercise for humanity is to desire to know the truth, and then seek it! Most people haven't spent the kind of time seeking it that they do amusing themselves, alas, to their detriment. But no one can force this upon anyone either. Islam does try however.
Let's assume that one or more of these religions, I'll call them "faiths," is correct. Well, that necessarily then means that the others are incorrect or wrong as a result. Think about it. If Christianity is proven right, then Islam and any other religion/faith that runs counter to it in any way, must be false. Simple logic.
The converse also applies. Suppose we knew as a fact that Islam were correct. The the God that Jews and Christians believe in would have to be a mental machination, because otherwise it wouldn't support what we know to be true.
So the exercise, for those willing to undertake it, is to engage in the search for "Who God is?" How to undertake that is without prescription. What matters is the determination that every person comes to. That however is not to say that simply because different people come to different determinations, that all are equally valid as our world/society often attempts to do, for the reasons stated above.
In attempting to validate, or otherwise invalidate, certain faiths, the determining factor must be the credibility behind each. What drives it?
In the case of Islam, it is Mohammed. He wrote all of Islam's underpinning documents. What he wrote clearly had an influence from the Bible and the Torah. After that, it was his opinion coupled with things that he claimed he garnered in a vision that he had involving the Angel Gabriel. He later even questioned his own visions and thoughts as to whether they were demonic or not. People choosing to believe in Islam, necessarily take on as a belief what Mohammed taught.
In the case of Christianity, Jesus came to earth, claimed to be God's son, took on our sin in essence, and then paid the penalty of sin for those that would accept his payment for it, and for a penalty that the Bible states is separtion from God, or spiritual death. Christ did many, many things in front of many, many people, and his life was openly on view for all to see. He then died (was crucified) and ascended to heaven, visibly.
I won't go into the others and will leave them, if you like, as an exercise for yourself.
Regardless, it is up to each person to take in the facts, or not, and come to a determination within him/herself. We live life, and then either pass into heaven as a result of Christ's "payment" for our sins and transgressions which have been residual from the "fall of man" and are inherent in every person born, or we stand before him to account for our lives and hear His reconciliation of our life and to bear the burden of making such payment ourselves, via a separation from Him. But remember, He is the one that created us and that includes all of the good things about us. He is a good God.
I am assuming that you are skeptical, which is what life is for. If the afterlife concerns you, as it should, then I would challenge you to look into matters more. But the bottom line for you and for everyone else, is that you do your personal due diligence and see for yourself.
Hint: Start with Christianity and work "backwards." Christianity is unique in numerous ways and there's a reason for that. Remember as well, ask yourself throughout your "seeking" process what the basis and foundation for each religion/faith is, and consider those very, very strongly.
Another hint: Does it come from God Himself? Or does it evolve from a man, group of men/people, etc. claiming to "have the corner," as it were, on god? That's incredibly important.
I could write volumes here. It would all only lead you to the proverbial water however. Your process will aid you in validating things while invalidating others. If you dig down and avoid superficial discovery, I believe that you will be amazed at how incredibly easy the choice is over time. Many, many very intelligent people over time have taken up the task of vehemently challenging Christianity by setting out to disprove it only to have had them find their way bowing to the Living God!
I do want to point out however, that your premise here could not be more flawed, which brings us full circle on this.
One more point that just hit me.
In the Bible, Luke Chapter 1, there is an account of the Angel Gabriel in essence validating Christianity.
Yet, Mohammed claims that that same angel told him otherwise, again, in essence.
Clearly, your exercise has just been shortened, you can eliminate one of the two faiths from your options.
Another Hint: Christ came and walked the earth about 600 years before Mohammed was even born. His status as the Sone of God was fully validated by Gabriel. Read for yourself.
It's easy to see where Mohammed got the basis for his material however.