There is a difference between everything and everything essential.
Again, I'll reference Polycarp, student of Paul, who wrote a letter to the Philippians. In that letter he states that Paul "wrote a letter unto you, into the which if ye look diligently, ye shall be able to be builded up unto the faith given to you,"
Polycarp was a disciple of the Apostle Paul. He says that just one letter of Paul is sufficient to build up the faith. Imagine what all of the letters of the new testament make possible?