I find that most Christians don't understand their own scriptures. Yes, that verse is "God" speaking. We know this from its context starting at 21:5
Rev. 21:5
And he that sat upon the throne said, Behold, I make all things new. And he said unto me, Write: for these words are true and faithful.
We know that ("he that sat upon the throne") refers back to Rev. 4 and 5 where the "Lamb", who Christians refer to as Jesus (Yeshua), and his god are viewed separately by "God's" heavenly creation. This is substantiated by John himself later in Rev. 21. In 4 and 5 the one on the throne is identified as "God" while the Lamb (Yeshua) was "Standing" amongst the 24 elders.
Rev. 21:22
And I saw no temple therein: for the Lord God Almighty and the Lamb are the temple of it.
John continues to inform the reader that both entities are separate.
Some KJV Bibles print this in red and some in black. Red is correct. The way to find out here who is speaking is to check the context. Revelation 21:6
says, "And he said unto me, It is done.
I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely."
The one speaking here is the Alpha and Omega, the beginning and the ending.
As you say, this is God the Father speaking, the Alpha and Omega, the Almighty.
Revelation 1:8
(8)
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.
(Again some bibles use red and some black. Red is correct as we shall see.)
Prepare to meet Jesus, the Alpha and Omega, the *** Lord God Almighty ***:
Revelation 1:11-18
(11) Saying,
I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.
(12)
And I turned to see the voice that spake with me. And being turned, I saw seven golden candlesticks;
(13) And in the midst of the seven candlesticks
one like unto the Son of man, clothed with a garment down to the foot, and girt about the paps with a golden girdle.
(14) His head and his hairs were white like wool, as white as snow; and his eyes were as a flame of fire;
(15) And his feet like unto fine brass, as if they burned in a furnace; and his voice as the sound of many waters.
(16) And he had in his right hand seven stars: and out of his mouth went a sharp twoedged sword: and his countenance was as the sun shineth in his strength.
(17)
And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me,
Fear not; I am the first and the last:
(18) I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
This is Jesus Christ, the God of the Bible:
Isaiah 9:6
(6)
For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor,
The mighty God, The everlasting Father, The Prince of Peace.
It all lines up.
