‘I am the Alpha and the Omega,' says the Lord God, ‘who is and who was and who is to come, the Almighty.'
At this point God is seen as dramatically stepping in to make His declaration over the whole revelation, reinforcing John's words in Revelation 1:4.
Alpha and omega are the first and last letters of the Greek alphabet. Thus He is declaring Himself to be the beginning and the ending, the One Who sums up everything in Himself from start to finish. But there may also be the idea that every letter in this revelation comes directly from Him. That each letter is rooted in Him (compare Revelation 22:18-19), as is all that happens.
He is ‘the Almighty'. The word means the all-powerful One, the Omnipotent One. In the Septuagint it translates ‘the God of hosts', the One Who is over all that is (Hosea 12:5; Amos 3:13; Amos 4:13; Amos 5:14). He is the One Who ‘forms the mountains and creates the wind, and declares to man what is his thought, who makes the morning darkness and treads on the high places of the earth, the Lord, the God of hosts is his name' (Amos 4:13).
He is also the ever existing One Who is there with His people, the One Who always was, the One Who always will be. As ‘the One Who is' He controls history and destinies, as ‘the One Who was' He created all things and fashioned history, as the One Who ‘is coming' He sums up the future. And He is the Almighty (compare 2 Corinthians 6:18). All things are in His hands. So as the people of God face up to what is to come they can rest in the confidence of the overall power of their protector, the ruler of time and of history and of all that is and will be.