Who is worthy to open the seven sealed book?
Only Jesus Christ, called the Lion of the tribe of Judah and the Root of David, was found worthy to open the sealed book and loose its seven seals.
When a strong angel cried out asking who was worthy to open the sealed book and loose its seals, no one in heaven, on earth, or under the earth could be found worthy. This caused John to weep bitterly, because it seemed the book would remain forever closed and its contents forever hidden.
But one of the elders told John not to weep, for the Lion of the tribe of Judah, the Root of David, had prevailed to open the book and loose its seven seals. This title reaches back to the promise that kingship and authority would belong to Judah's line until the coming of the rightful ruler, and that promise pointed to Jesus Christ. Through both His legal lineage and His bloodline, Jesus descends from Judah, which fully qualifies Him as the Lion of that tribe.
Jesus proved worthy not merely by lineage but because He is the Lamb who was slain, and upon receiving the book from the right hand of God, He alone was able to take it and begin opening its seals one by one. No other being, past, present, or future, could be found to do this. Only Christ, in His unique worthiness as both Lion and Lamb, could receive the revelation of things to come and unveil it to John.
The scriptures
And I saw a strong angel proclaiming with a loud voice, Who is worthy to open the book, and to loose the seals thereof? And no man in heaven, nor in earth, neither under the earth, was able to open the book, neither to look thereon. And I wept much, because no man was found worthy to open and to read the book, neither to look thereon.
And one of the elders saith unto me, Weep not: behold, the Lion of the tribe of Juda, the Root of David, hath prevailed to open the book, and to loose the seven seals thereof.
And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth. And he came and took the book out of the right hand of him that sat upon the throne.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.