Commentaries:
Whatever this glory is that He asks to be restored, it is something He did not have as a human, but He did have when He truly was fully God. He had it before He was born of Mary, did not have it during His physical life, and had it returned to Him upon His resurrection and ascension.
In the New Testament, glory is used in the sense of anything that brings honor and praise upon a person. It can be one's works, attitude, manner of living, skill, strength, wisdom, power, appearance, or status. Some or all of these could be included within the framework of Christ's request. The Bible does not clarify or expand on what He specifically meant, but whatever it was, it was lacking in Him while He was human. Therefore He could not have been "fully man and fully God."
John W. Ritenbaugh
Fully Man and Fully God?
The first thing Christ does in this prayer is establish that He was with the Father. In this case, the word with means "beside" or "alongside of." This agrees with John 1:1, "In the beginning was the Word [Christ], and the Word was with [along side of] God, and the Word was God."
John W. Ritenbaugh
The Nature of God: Elohim
Other Forerunner Commentary entries containing John 17:5:
Genesis 18:1-8
Matthew 6:9-13
Matthew 6:9-13
Matthew 26:39-44
Luke 12:47-48
John 1:1-3
John 17:11
1 Corinthians 15:49