Title of Christ in Apocalypse 1, 5.
The glorified Christ in heaven fulfills the function of witness of what God the Father wills, as He fulfilled it on earth.
Saint John's Gospel 18:37, teaches that Christ gives witness of the truth which He had seen in the bosom of the Father; the Apocalypse represents Him as giving witness of God's designs regarding the future.
In Apocalypse 3:14, He is called "the faithful and true witness," i.e., one who may be trusted and whose testimony never falls short of the truth.
New Catholic Dictionary