Summary: There are many today who are confused about Jesus. The most effective testimony in any court is an eye witness. John is an eye witness and you heard his testimony today.
In the beginning of the church when the original apostles were teaching and planting churches, including Paul, the New Testament as we have it today had yet to be compiled. The writers of the New Testament were all first generation Jesus followers and their work would not be complete until John finished his revelation almost a hundred years after Jesus resurrection. But when Paul, Peter, John, Luke, and others were writing the inspired Scripture there were heresies and lies being passed around the churches.
There were three big lies that the New Testament writers address. The first lie was that Gentile Christians had to become Jews before they could become Christians. Paul went to great lengths to explain that this was not true. He even confronted Peter who had to repent of this lie. John doesn't address this particular heresy in First John, probably because Paul did a really good job of exposing it in his letters and teaching.
But the book of First John does expose the following lies that became significant and wide spread after the apostles were dead.
The second lie was called Gnosticism. This false teaching in a very simplified statement said that Jesus wasn't really human but an illusion, therefore, he didn't die because he was never really human. The Hebrew writer spoke to this when he said, "He was tempted in every way like we are." In other words Jesus was a human being not just an illusion.
The third lie John labeled as the Nicolaitans. This word is a combination of three Greek words that basically mean, "victory over people." Jesus is the victory, John will respond in this book. This falsehood taught that only certain people had access to God. Those who spread this heresy were abusive of their perceived power and many manipulated Christians who didn't know better for financial gain. Some of these folks even claimed to be apostles just like Paul, Peter and John. Peter addressed this when he said "Don’t lord it over the people assigned to your care, but lead them by your own good example."
John was an elder in the church in Jerusalem but after Paul's execution there was a leadership void in Asia Minor. The churches that Paul planted during his missionary career were floundering even though they had received further teaching from Paul through his letters. John was the last apostle to survive and it fell on his shoulders to shepherd the churches that Paul had planted. John set up shop in Ephesus, the same church where Timothy preached.
John summarized the state of the seven churches in Asia Minor in his Revelation letter. The first church he describes through the voice of Jesus is his home church in Ephesus; "...You have examined the claims of those who say they are apostles but are not. You have discovered they are liars. You have patiently suffered for me without quitting. But I have this complaint against you. You don’t love me or each other as you did at first! ...But this is in your favor: You hate the evil deeds of the Nicolaitans, just as I do..."
John had to be despondent to hear Jesus talk about his church losing their love for Jesus and for each other. I also believe he would've taken satisfaction in hearing Jesus commend this church for their diligence to discover the truth and expose the lies of false teachers. That would've been because of the work John did with the church in Ephesus.
Through the book of First John God defines the Jesus movement. There were many confused people in the first century, not sure of what the truth really was. Was Jesus human? Were his apostles really inspired by God? Could only certain people communicate with God and did they have spiritual authority to forgive sin? Did I really have to be a practicing Jew in order to be a follower of Jesus? There was a need to provide a definitive statement to address these questions and First John is that source. If you're a Christian but you struggle with who is telling the truth about God stuff, then, read this book. John wrote it for this reason. We're going to walk through the book of First John one chapter a week. In each chapter of the book John defines these characteristics of the movement, chapter 1- Jesus, chapter 2- Faith, chapter 3- Love, chapter 4- Church, chapter 5- Eternal Life.
So today John defines for those who might be confused about who Jesus is, especially for those who struggle with his humanity and divinity in Chapter One.
Try to imagine John in a courtroom on the witness stand. The jury is listening intently, the judge is leaning forward, and the room is packed with people all wanting to hear John's answer to the question the prosecutor has just asked. John is not only the last remaining apostle alive, but, he was the best friend of Jesus. He was the inner circle. According to John, Jesus loved him more than he loved anyone else. No human being knows Jesus better than John. This is why John has credibility. He walked with Jesus, talked with him, witnessed the most incredible things human eyes have ever seen with Jesus, including a dead guy raised from the dead, five loaves of bread and two fish multiplying itself enough to feed thousands of hungry people, and Jesus walked on water. John was there. He could tell you every little detail about what happened, and Peter's epic fail, of course, John would always get a laugh out of that.