Religion is a system of belief with a core belief at the center, a reason for all the other beliefs, the central point on which every other belief is balanced. It is not true that the other beliefs can be cast aside or kept depending on our personal preference, but they remain peripheral to the central belief. The other parts of religion may be very important, but everything starts with one act of faith.
For Judaism, the central belief is a promise made to Abraham, fulfilled in Moses. If it were discovered that Moses was actually named Tom, that would not change the Jewish religion, but if it were proven that the Lord did not save the descendants of Abraham from slavery in Egypt then the whole faith would be shaken.
For Christians, our central belief is in a person, the person of Jesus Christ. Sometimes people try to shock us by showing that Jesus was actually born in March, but whether he was born in winter or springtime is not nearly as important as that he was born. God, the creator of the universe, was born of a woman. For our sake, he became one of us, a human. The central fact of Christianity contains the life, death, and resurrection of Jesus Christ. If that is true, then everything else can be important in its own way. If that is false, none of the other stuff matters.
This is what St. John is saying in his letter. He says that anyone who denies that Jesus is the Christ is a liar. Perhaps this statement is not the most ecumenical or interreligious, but the central fact must either be true or false. We cannot follow a religion if the central idea of the religion is a lie. Christmas might have Christmas cookies and a Christmas tree and Christmas presents, but if Jesus were not born, there is no point in having Christmas at all.
St. John knew Jesus personally. He spoke with him; he ate with him; he followed him. He wants to express to us that the central fact of Christianity is not merely an intellectual idea that some wise person thought up. At the core, it is about a man he knew, who lived, who died, yet who still lived. He was not only a man. He was God. That is the kind of fact that can change everything.