Before Christ came into the world, He dwelt in heaven with the Father. When the angels looked upon the Lord, they saw all the glory of Deity. To every eye, He was obviously God. But when He came among men, the glory of Deity was veiled. Though He was still God it was not apparent to most onlookers.
They saw Him merely as the carpenter’s Son. Here, the Savior is praying that the visible manifestation of His glory in heaven might be restored. The words “glorify Me together with Yourself” mean ”glorify Me in Your presence in heaven. Let the original glory which I shared with you before My Incarnation be resumed.” This clearly teaches the pre-existence of Christ.
—William MacDonald, Believer’s Bible Commentary, p 1496