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..."