In Revelation 1:5; 3:14, the faithful witness signifies that the Lord is Divine Truth itself, and the Lord as the Word, which is divine truth from Him. (Apocalypse Revealed 16, 199, Apocalypse Explained 226)

199. "'These things says the Amen, the Faithful and True Witness.'" This symbolizes the Lord in relation to the Word, which is Divine truth originating from Him.

"Amen" is a Divine confirmation springing from the truth itself, which the Lord is, and so which is from Him, as may be seen in no. 23 above. Moreover, a faithful and true witness, when applied to the Lord, is the Divine truth which originates from Him in the Word (nos. 6, 16).

Whether you say that the Lord testifies of Himself, or that the Word does, the meaning is the same, since the Son of Man, who is the one addressing the churches here, is the Lord in relation to the Word (no. 44).

This declaration is made first to this church, because the subject here is people in the church who base their beliefs both on their own thinking and on the Word, and people who base their beliefs on the Word, form them from the Lord.