Dating the gospels
It would perhaps be best to first establish the case that the same author is responsible for all the books associated with John.
The New Testament books of John, 1 John, 2 John, 3 John and Revelation are sometimes called the Johannine literature and are traditionally assigned to John the son of Zebedee, one of the twelve disciples of Jesus.
Jesus says to the sick man, Do you wish to get well?
The sick man replies Sir, I have no man to put me into the pool when the water is stirred up, but while I am coming, another steps down before me" (John 5:6-7).
2 John says Though I have many things to write to you, I do not want to do so with paper and ink; but I hope to come to you and speak face to face, while 3 John 13-14 has the remarkably similar I had many things to write to you, but I am not willing to write them to you with pen and ink; but I hope to see you shortly, and we will speak face to face.
In both books the author rejoices over children walking in truth.
Both 1 and 2 John speak of a new commandment (1 John 2:8, 2 John 5) of love.
Truth is a key concept in all three (1 John 1:6, 1:8, , , 4:6, 5:6; 2 John 1, 2, 4; 3 John 1, 3, 4, 8, 12).
Many of these themes in 1-3 John are also present in the Gospel of John.The subject of truth and the idea of a commandment of love is prominent in both books, along with the idea that God is light."Eternal life" is a phrase that occurs with disproportionate freqency in John and 1 John.Ignatius (35-107), Papius, Iraneus and Origin (185-254) assigned John the son of Zebedee as the author of the Gospel of John.However, Papius identifies a separate John as the writer of the letters of John and Revelation, so there is some variance in early tradition as to authorship of the Johannine letters.