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.
The unconditioned priority of God in His universe is a truth celebrated both in the Old Testament and in the New. The prophet Habakkuk sang it in ecstatic language, “Art thou not from everlasting, O LORD my God, mine Holy One?” (1:12). The Apostle John set it forth in careful words deep with meaning,
In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made (1:1-3)….
Here we acknowledge (and there is fear and wonder in the thought) the essential unity of God’s nature, the timeless persistence of His changeless being throughout eternity and time…. Begin where we will, God is there first. He is Alpha and Omega, the beginning and the ending…. If we grope back to the farthest limits of thought where imagination touches the pre-creation void, we shall find God there. In one unified present glance He comprehends all things from everlasting, and the flutter of a seraph’s wing a thousand ages hence is seen by Him now without moving His eyes. POM001, 003-004
Your timelessness is a truth that is beyond my comprehension, Lord, but it leads me to fall before You in awe. Amen. 
The Certainty of the Second Coming
“I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.” (1:8)
In this verse the Lord God puts His signature on the prophecy of the Second Coming recorded in the previous verse. Three of His divine attributes guarantee the certainty of the pledge of Christ’s return.
Alpha and the Omega emphasizes God’s omniscience. Alpha is the first letter of the Greek alphabet, and Omega is the last. All knowledge is conveyed through the letters of the alphabet; thus God’s designation of Himself as the Alpha and the Omega affirms that He has all knowledge. He knows, therefore, the certainty of this promise.
As the one who is and who was and who is to come, God’s transcendent, eternal presence is not confined by time or space or any feature or event in them. There is no possible contingency of which He is unaware regarding the Second Coming. Thus, His promise that the Lord Jesus Christ will return settles the issue.
The designation of God as the Almighty (cf. 4:8; 11:17; 15:3; 16:7, 14; 19:6, 15; 21:22) affirms His omnipotence. Since He is all powerful, nothing can hinder Him from carrying out His sovereign will. No one or no thing can possibly prevent Christ from returning in glory as described in verse 7.
Jesus came the first time in humiliation; He will return in exaltation. He came the first time to be killed; He will return to kill His enemies. He came the first time to serve; He will return to be served. He came the first time as the suffering servant; He will return as the conquering king. The challenge the book of Revelation makes to every person is to be ready for His return.
John Phillips writes,
One of the most stirring pages in English history tells of the conquests and crusades of Richard I, the Lionhearted. While Richard was away trouncing Saladin, his kingdom fell on bad times. His sly and graceless brother, John, usurped all the prerogatives of the king and misruled the realm. The people of England suffered, longing for the return of the king, and praying that it might be soon. Then one day Richard came. He landed in England and marched straight for his throne. Around that glittering coming, many tales are told, woven into the legends of England. (One of them is the story of Robin Hood.) John’s castles tumbled like ninepins. Great Richard laid claim to his throne, and none dared stand in his path. The people shouted their delight. They rang peal after peal on the bells. The Lion was back! Long live the king!
One day a King greater than Richard will lay claim to a realm greater than England. Those who have abused the earth in His absence, seized His domains, and mismanaged His world will all be swept aside. (Exploring Revelation, rev. ed. [Chicago: Moody, 1987; reprint, Neptune, N.J.: Loizeaux, 1991], 22–23)
Only those “who have loved His appearing” (2 Tim. 4:8), who love Him and acknowledge Him as the rightful king, will enjoy the blessings of His kingdom.
8 Such a stupendous promise requires more than the prophet’s own signature or even Christ’s “Amen.” God himself speaks and, with his own signature, vouches for the truthfulness of the coming of Christ. Of the many names of God that reveal his character and memorialize his deeds, there are four strong ones in this verse: “Alpha and Omega”; “Lord God”; “who is, and who was, and who is to come”; and “the Almighty” (cf. v. 4 for comments on the second title). Alpha and omega are the first and last letters of the Greek alphabet. Their mention here is similar to the “First” and “Last” in v. 17 and is further heightened by the “Beginning” and the “End” in 21:6 and 22:13. Only the book of Revelation refers to God as the “Alpha and the Omega.” God is the absolute source of all creation and history; nothing lies outside of him. Therefore, he is the “Lord God” of all and is continually present to his people as the “Almighty” (pantokratōr, lit., “the one who has his hand on everything”; GK 4120; cf. 4:8; 11:17; 15:3; 16:7, 14; 19:6, 15; 21:22; 2 Co 6:18).
 Tozer, A. W., & Eggert, R. (2015). Tozer on the almighty god: a 365-day devotional. Chicago, IL: Moody Publishers.
 MacArthur, J. F., Jr. (1999). Revelation 1–11 (pp. 34–35). Chicago: Moody Press.
 Johnson, A. F. (2006). Revelation. In T. Longman III & D. E. Garland (Eds.), The Expositor’s Bible Commentary: Hebrews–Revelation (Revised Edition) (Vol. 13, p. 601). Grand Rapids, MI: Zondervan.