Of all religions in the world, which, if any, is true? This question is important and so is the answer, but it is not as difficult as one might think. In fact, we can begin by looking at how to recognize what is false because this can help us get rid of a host of theological systems that, well, just don't work.
There are two main ways to determine if something is not true: internal and external. By internal we mean if a belief is internally inconsistent, i.e., self-contradictory, then it cannot be true. By external means we mean that if a belief is contradicted by a fact, then the belief cannot be true.
Therefore, if we were to find religions that fit either of these two criteria we would know which religions cannot be true by simply applying logic and evidence to them. But finding out which ones are not true does not mean we have found one that is. Let's now turn our attention to truth.
Truth is that which corresponds to reality. Also, truth is a statement. For example, you are reading this sentence. This statement is true and it also corresponds to reality. If there are no statements, there are no truths. Does that sound confusing? It isn't. First of all, we must not confuse truth statements with mere existence. If the only thing that existed in the universe was a carbon sphere, its existence would be a fact. But without someone making a statement about its existence, the truth or falsity of that statement could not be determined. Therefore, truth deals with statements and statements require a mind.
This is useful because if something cannot be determined to be true because it is a belief system that cannot be verified or unverified, its truth value is nil. Here's an example. Back in the 1990s in San Diego California, a group of people committed mass suicide in the Heaven's Gate cult. They taught that on the other side of an approaching comet was a spaceship that would take their souls away if they were to die. So, they all killed themselves.
The problem was that these assertions were not verifiable. There was no means by which to verify if there was or was not a spaceship on the otherside of the comet, nor was there a way to verify that their souls would be taken away by this supposed spaceship. So, their belief system was not falsifiable. That is, it was not able to be shown to be false -- or true. Usually when a belief system is not falsifiable it is because the belief system is not based in reality. It is nothing more than an intellectual invention.
Let's cut to the chase
Jesus said, "I am the way, the truth, and the life, and no one comes to the Father but by me," (John 14:6).
Instead of looking at thousands of religious systems and finding internal inconsistencies, facts that contradict the belief systems, or try and determine whether or not each is falsifiable, let's take a look at Christianity. Why? Because Jesus said that he alone was the way to God the Father. If Jesus is right, then all other religions are false since all other religions don't agree with what Jesus said.
If it could be shown that there were other ways to God the Father besides Jesus, then Jesus' statement would have been falsified and Christianity wouldn't be true. This would mean that Jesus was wrong and his claims would likewise be wrong. Jesus' statement is all the more important when we realize the source documents concerning Jesus' life (the gospels of Matthew, Mark, Luke, and John) are eyewitness accounts of Jesus' life both before and after his crucifixion and resurrection. Why is this important?
It is important because the gospels are not hearsay or legend. They are eyewitness recordings of Jesus' life, miracles, claims, and teachings. If Jesus claimed to be the only way to God the Father, then we need to look at the evidence so we might be able to determine if what he said is true or not. Such an incredible claim would need some incredible evidence to back it up. So, let's look at the evidence...in a moment.
First, we have to make a point. It is not possible to prove beyond a shadow of a doubt that certain ancient historical events have occurred (since filming technology was not existent then). So, like any historical event, we examine the evidence and draw logical conclusions based on the evidence. After all, proof is reserved for logic and mathematics. Historical events are not subject to proof tests in logic and math, but they are subject to various evidences and logical examination.
First of all, the New Testament Gospels were written before the end of the first century. The book of Acts is highly important in this regard since it is a written record of the historical events of the early church. Acts does not contain the significant date markers, such as the destruction of Jerusalem in 70 A.D., the persecution of Christians by Nero in 64 A.D., or the deaths of the apostle Paul and James each in 62. A.D. Therefore, it is logical to conclude that the book of Acts was written no later than 62 A.D. - otherwise it would have recorded these highly significant events. So, let's give the book of Acts a date of say 60 A.D.
Acts was written after the book of Luke, and according to most scholars Luke was written after Matthew which was probably written after Mark. Therefore, we have good evidence that the Gospels were written before 62 A.D., and since Jesus was crucified in 33 A.D. that means we have less than 30 years difference between the life of Christ and the records being written down by those who were there. This also means there's not enough time for legend to creep into the accounts of Jesus' life, death, and resurrection. This is what is so significant.
The Gospels contain the eyewitness accounts of the life of Christ. Since Jesus said that he alone was the way, the truth, and the life (John 14:6) -- which is an incredible claim -- we then look to the miraculous events surrounding him that support his claim. It is one thing to claim to be the only way, it is another to demonstrate it. This he did by demonstrating some miraculous and unique abilities. First, Jesus fulfilled many prophecies written in the Old Testament. Second, he performed many miracles recorded in the New Testament. Third, he claimed to be God in flesh (John 8:24; 8:58 with Exodus 3:14). Fourth, he died on the cross and physically rose from the dead (1 Cor. 15:1-4; Luke 24:39; John 20:27-28)...according to the eyewitness accounts.
This means that Jesus backed up his claims with miracles. Has anyone else in any other religion fulfilled prophecy, performed miracles, claimed to be God, died and rose from the dead? Nope. Jesus is the only one.
Therefore, Jesus' claim that he is the only way to God is true and this means that all other religions are not.