This is the disciple who testifies to these things and who has written them down: John’s Gospel is based on his personal experiences. It is not a story written from rumors or guesses, but from the memories of a man who spent important years with Jesus. He remembered, with the help of the Holy Spirit, what Jesus said and did (14:26).
we know: This account of the life of Christ was not a guess or from weak memories. Instead, John based it on the confident knowledge of reliable eyewitness accounts.