And He said to me, "IT IS DONE.
From the previous verse, we know that the 'He' speaking here is the Lord God, the Almighty (Revelation 1:8 and Revelation 4).
'IT IS DONE' signifies that the transition from the old earth/heaven to the new earth/heaven is DONE (verse 1). Just like the old creation was DONE (Genesis 2:1-3) and the work of redemption on the old earth is DONE (John 19:28-30), now the transition from old earth/heaven to new earth/heaven is DONE.
I am the Alpha and the Omega, the BEGINNING and the END.
This looks like a conundrum: the new earth/heaven is eternal with no BEGINNING and no END, and yet we have a reference to a 'BEGINNING' and an 'END'.
What we are seeing is that what Christ did in the old earth/heaven (Revelation 1:8) will transition through to the new earth/heaven. Our redemption did not stop when the old earth/heaven did (next verse). PRAISE HIS NAME FOREVER, AMEN!
From old earth physics, we know of 4 dimensions: 3 of length and 1 of time. Clearly in verse 1, we see the transition of the 3 length dimensions. And here we can say that we see the transition of time from finite (Alpha - Omega, BEGINNING - END) to the infinite. Only God who is infinite knows how this happens; we can only experience it (and I can't wait).
I will give to the one who THIRSTS from the spring of the WATER OF LIFE without cost.
This is the same kind of THIRST described in Psalm 42:1-2. It is a THIRST for the GOOD WATER OF LIFE provided by Jesus Christ (1 John 5:6-12 and John 4:7-24). We see a connection to the presence of the Holy Spirit in this GOOD WATER OF LIFE. Only in this chapter and in the next (Revelation 22:1,17) do we find reference to this WATER OF LIFE. Generally, the WATER OF LIFE is withheld from old earthlings except for those overcomers (next verse) who accept Jesus Christ's redemptive work. This will be the most precious commodity in the new earth/heaven on the other side of the transition.
When we take a broad look at this verse, we see confirmation of the HOLY TRINITY in the new universe. We see the Lord God, the Almighty, as speaker; Christ as Alpha and Omega; and Holy Spirit within the WATER OF LIFE.