For the record, I am a Trinitarian.
Textual variants
Textus Receptus:
Ἐγώ εἰμι τὸ Α καὶ τὸ Ω ἀρχὴ καὶ τέλος, λέγει ὁ κύριος ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος ὁ παντοκράτωρ
NA27:
Ἐγώ εἰμι τὸ ἄλφα καὶ τὸ ὦ, λέγει κύριος ὁ θεός, ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος, ὁ παντοκράτωρ.
There is a difference between the Textus Receptus and NA27. I searched Constantine Tischendorf's critical apparatus on Revelation 1:8 to identify the significant textual variants and witnesses.
Notes:
- NA27 omits ἀρχὴ καὶ τέλος ("[the] beginning and [the] end"). Witnesses that include the phrase are:
Novum Testamentum graece. 1869-94 (C. v. Tischendorf, C. R. Gregory & E. Abbot, Ed.) (Re 1:8). Lipsiae: Giesecke & Devrient.
- NA27 has κύριος ὁ θεός ("Lord God") which is the equivalent of the Hebrew יְהוָה אֱלֹהִים. cp. LXX Gen. 2:15. Textus Receptus has ὁ κύριος ("the Lord") but omits ὁ θεός ("God"). Most witnesses appear to include ὁ θεός like the NA27:
Now, the answer to your question. Who is referred to as the Alpha and the Omega in Revelation 1:8?
Exegesis
Beginning with Rev. 1:4 (KJV),
John to the seven churches which are in Asia: Grace [be] unto you, and peace, from him which is, and which was, and which is to come; and from the seven Spirits which are before his throne;
Notice in v. 4 that it refers to "Him who is, and who was, and who is to come." In the Greek, it is ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος. This is a very important clue to who is talking in v. 8.
Now, in Rev. 1:5 (KJV),
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,
Put vv. 4-5 together:
John to the seven churches which are in Asia: Grace [be] unto you, and peace, from him which is, and which was, and which is to come; and from the seven Spirits which are before his throne; 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,
Observe that "He who was, and who is, and who is to come" is distinguished from "Jesus Christ, the faithful witness, the first begotten of the dead, and the prince of the kings of the earth."
Therefore, "He who was, and who is, and who is to come" is God the Father, not Jesus Christ.
John Gill on Rev. 1:4:
But rather this is to be understood of the first Person, of God the Father; and the phrases are expressive both of his eternity, he being God from everlasting to everlasting; and of his immutability, he being now what he always was, and will be what he now is, and ever was, without any variableness, or shadow of turning: they are a periphrasis, and an explanation of the word "Jehovah", which includes all tenses, past, present, and to come.
Rev. 1:6 is John referring to Jesus who "made us kings and priests to God and his Father":
And hath made us kings and priests unto God and his Father; to him [be] glory and dominion for ever and ever. Amen.
Rev. 1:7 is John describing Jesus coming upon the clouds of heaven (cp. Dan. 7:13-14):
Behold, he cometh with clouds; and every eye shall see him, and they [also] which pierced him: and all kindreds of the earth shall wail because of him. Even so, Amen.
Finally, in Rev. 1:8 (KJV):
I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.
But, Rev. 1:8 (NA27):
"I am the Alpha and the Omega," said the Lord God, "who is, and who was, and who is to come, the Almighty."
Now, it really doesn't matter which translation you use. Rev. 1:4 already distinguished between "He who is, and who was, and who is to come" and "Jesus Christ." Thus, the person speaking is God the Father, not Jesus Christ.
Elsewhere in Revelation (e.g. Rev. 22:13), Jesus Christ is identified as the Alpha and the Omega, but not here in Rev. 1:8.
