He is called the faithful witness,
which means that he gives accurate testimony to who God is (see John 1:18) and to the revelation God gave to him to show his servants (Revelation 1:1). So he is witness to the world that the Lord alone is God, as the actions he promises to perform in coming chapters will illustrate.
5 and from Jesus Christ the faithful witness, the firstborn of the dead, and the ruler of kings on earth.To him who loves us and has freed us from our sins by his blood