1 John 1:2
 the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us— (ESV)
John had a tall order on his hands. He is refuting wrong teaching about the nature of Jesus Christ. In this verse he re-states what is the truth about Jesus Christ. His life was made manifest, it was revealed. This life of Jesus was seen and testified to by real men, including John himself! But John makes very clear that this life was both eternal, and yet made manifest. He is saying that Christ is both eternally God, and yet he became man.
This is a hard concept. Although truth does not cease to be truth because it is difficult. The Jews are asked to believe that God came down to earth and was born of a virgin, Mary. That's a lot to ask. However, if the Jews had read their own prophecies about the Messiah they would have seen that Jesus' coming to earth was the very fulfillment of all that they professed to believe. The great C.S. Lewis put it this way:
"Among the Jews there suddenly turns up a man who goes about talking as if He as God… Now let us get this clear. Among Pantheists… anyone might say that he was part of God, or one with God: There would be nothing very odd about it. But this man, since He was a Jew, could not mean that kind of God. God, in their language, meant the Being outside the world who made it and was infinitely different from anything else. And when you have grasped that, you will see that what this man said was, quite simply, the most shocking thing that has ever been uttered by human lips.
- C.S. Lewis
The Apostle John takes that shocking claim and makes it personal. Yes, it is shocking to say that Jesus was God, but John says, "I walked with him. I talked with him. I loved him. I saw his miracles. My life was changed." There is one thing to talk about things theoretically. It is quite another to speak from experience.
God is eternal and yet he became flesh for us. Is this truth theoretical for you? Or have you experienced the life-changing salvation of Jesus Christ?