The trouble with Christianity is that Satan and his forces will do anything they possibly can to keep man from accepting it, or tear man out of it, up to and including creating counterfeit versions of the faith. We are often told that all one needs to be a Christian is to claim it. Even I remember, as a young Christian, having a much stronger trust in any source claiming to be Christian. In fact, you may be surprised to know that there was a time in my life when I considered the Roman Catholic Church to be a Christian organisation, just like any other denomination.
This is actually the definition accepted by the secular world. If you claim to believe in Jesus, you are automatically considered Christian, no matter how many heresies you believe and profess, no matter how many essential doctrines you cast aside. You can literally be an atheist and the world can still find a way to consider you Christian! To this, Paul says be warned: There is such a thing as "another Jesus".
In the past I would use an analogy like the following: Imagine a man was born in 1990, and he started claiming to be Jesus Christ reincarnated. He gained a large enough following to be noted by scholars of religion, and his followers called themselves Christian. Would that be a legitimate title for them? The obvious answer is no. They're not Christian even though they're following a living and breathing man who claims to be Jesus Christ. The same is true for a Jesus of fiction. If you make up a false Jesus, and the result is the creation of a brand new denomination, it doesn't matter if you live in the 1990s or the 190s, you're a fake Christian.
As it happens, I no longer need to pose this analogy as a hypothetical. It turns out there have been a surprisingly large number of false Christs who have lived just since the 1800s. Arnold Potter (1804-1872), for example, claimed to be the reincarnation of Jesus Christ. He died while jumping off a cliff in an attempt to ascend to Heaven, after which his followers (yes, he had some) buried him. Lou de Palingboer (1898 - 1968) likewise claimed to be the reincarnation of Christ, gaining a moderate following. Dishonorable mention can be given to Ahn Sahng-hong who, while He himself did not claim to be Jesus Christ, nevertheless is the founder of a movement that claims him to be so. The World Mission Society Church of God, to this day, continues to preach Ahn Sahng-hong as the second coming of Christ, but now they also worship a new figure, God the Mother, whom they believe is alive in flesh.
Can any of the followers of these people be considered Christians in any meaningful sense? I can tell you this much: Whereas I consider there may be true believers in every denomination, I would have serious difficulty believing anyone can associate with these groups and call themselves Christians. For this reason, we can say that fictional versions of Jesus are likewise illegitimate. This leaves us with a scary question: Are we following one?
This is where the Bible comes in. All Scripture is given by inspiration of God, written down by men who were committed to this very task by Him. Some of these men personally saw Jesus die, after which they met Him alive again. Peter, for example, followed Jesus as His disciple right throughout His ministry, saw Him get arrested, denied Him before He reached the cross, and after He rose, was personally commissioned by Jesus: "Feed my sheep". Paul, while he does not seem to have seen Jesus die, and certainly didn't see Him before the ascension, nevertheless met the risen Lord on the road to Damascus, and was likewise commissioned to be the last Apostle. John, Matthew, these two were also disciples of the Lord. The entire New Testament is an authoritative source on the historical Jesus. It is the authoritative source on Jesus. Therefore, those who study it prayerfully and diligently shall never be lead astray by a false Jesus. Let us therefore never forsake our duty to study it.