Standing at the end of the pathway that had been given Him to tread, the Lord reviewed the thing that He had taught. Had His teaching been limited? If we were to put this query to ourselves we might answer, No; for He "in whom are hid all the treasures of wisdom and knowledge" may well be esteemed to have known no limitations. The Lord, however, reveals that He had been limited in the things which He had spoken- not to Himself, but by reason of the state of His hearers and the time during which they lived. Some things could not have been spoken until this occasion: "And these things I said not unto you at the beginning, because I was with you" (JOHN XVI: 4). Other things still had to remain unsaid, but the promise was given that "All Truth" should be revealed by the Spirit of truth that was to come.
I have yet many things to say unto you, but ye cannot bear them now. Howbeit when he, the Spirit of truth, is come, he shall guide you into all the truth: for he shall not speak of himself; but what things soever he shall hear, these shall he speak: and he shall declare unto you the things that are to come. He shall glorify Me: for he shall take of mine, and shall declare it unto you" (JOHN XVI: 12-14, R.V.).
Thus the Lord does not refer back to His own words as being the fulness of revelation, but rather directs the searcher for "All the truth" forward to things that were to be revealed after that Spirit of truth had come. All truth is not therefore to be discovered in the Gospels, but in the revelation given subsequently. While All Truth was not revealed by the Lord in Person when on earth, it nevertheless was to be concerning Him, and it would glorify Him.
from page 8, All Truth