John, the gospel writer known as the beloved apostle, writes three epistles to the Gentiles to assure them of eternal life based on his eyewitness account. John touches on light, truth, and love throughout his writing and affirms the truth of who Jesus is.
1 John deals with false teachers twisting who Jesus was, but John was there and saw and knew the truth. He opens his epistles by ensuring the reader that he has an eyewitness account based on the details he provides.