John the Apostle
Revelation 1:5ASV·traditional attribution

and from Jesus Christ, who isthe faithful witness, the firstborn of the dead, and the ruler of the kings of the earth. Unto him that loveth us, and loosed us from our sins by his blood;

Jamieson-Fausset-Brown Reformed

the faithful witness--of the truth concerning Himself and His mission as Prophet, Priest, and King Saviour. "He was the faithful witness, because all things that He heard of the Father He faithfully made known to His disciples. Also, because He taught the way of God in truth, and cared not for man, nor regarded the persons of men.

Geneva Bible Notes Reformed

And from Jesus Christ, [who is] the faithful witness, [and] the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood, (5) A most ample and honourable commendation of Christ, first from his offices of the priesthood and kingdom: secondly from his benefits, as his love...

Albert Barnes Presbyterian

Verse 5. And from Jesus Christ, who is the faithful witness. . He is faithful in the sense that he is one on whose testimony there may be entire reliance, or who is entirely worthy to be believed. From him "grace and peace" are appropriately sought, as one who bears such a testimony, and as the first-begotten from the dead, and as reigning over the kings of the earth.