WARNING: The following contains spoilers for One Piece, Chapter #1,001 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
Chapter #1,001 of One Piece contains a subtle but significant indication of how powerful Luffy has become, and it comes from none other than Kaido -- the Straw Hat captain's biggest adversary right now.
The moment happens during the aftermath of the blow Luffy just gave Kaido in Chapter #1,000. In Luffy's last fight with Kaido, none of his attacks seemed to damage the Yonko in the slightest, and when the Scabbards were facing off against Kaido it took all nine of them working together to even scratch such a powerful foe. But now, when Kaido falls to his knees from a single attack, he's reminded of the only other five individuals who could injure him to this extent, images of whom appear behind Luffy. Let's take a look at who these individuals are to prove that the rubber man is in amazing company, power-wise.
The first person on this list is Kozuki Oden, an important character in the Wano arc, and the only one of the five whose fight with Kaido we've actually seen. Oden was also responsible for giving Kaido his only visible scar -- the "X" on his waist. Kaido respected Oden till his last moments, so him seeing his greatest adversary in Luffy right now is nothing short of monumental.
The next person is Whitebeard. A meeting between Kaido and Whitebeard hasn't been shown in the manga thus far, but considering they used to serve under Rocks in the same crew when they were young, we're certain they've known each other for a very long time. When Whitebeard left the New World to save Ace from the Navy, Kaido planned on ambushing him and his men when their defenses were down, meaning Kaido realized Whitebeard wasn't a foe he could outright attack unless the opportunity presented itself.
However, Kaido never got to Whitebeard because another man was there to hold him at bay at the time of the War at Marineford: "Red-Haired" Shanks, the next person Kaido sees in Luffy. Kaido's relationship with Shanks is also a mystery, but we do know he and his crew were enough to stop Kaido from intercepting Whitebeard. In the end, Shanks made it to Marineford unharmed, meaning it's possible that he recognizes Shanks' strength or the two struck some kind of deal that made the Beast Pirates sail back to Wano.
Then there's Rocks. It's hard to believe someone as arrogant and ruthless as Kaido could have taken orders from anyone back in the day, but something about Rocks made him want to align with this chaotic pirate. He likely was not at his current level of strength back then, and Rocks could have seen his potential.
Gold Roger, on the other hand, who Luffy also reminds Kaido of, was someone who was able to fend off Kaido. During the God Valley Incident, Roger and Garp were able to defeat the Rocks Crew, which included Kaido, Big Mom and Whitebeard, as previously mentioned. Despite outnumbering Garp and Roger, Kaido was in the end, defeated by him, and the Rocks Pirates were dissolved after their captain's death.
But there's one other piece of information to take note of from the language Kaido uses when he refers to Luffy in Chapter #1001. Kaido says to him, "How high will your ceiling go?" It hasn't been very long since their last fight, and in just that short time, Luffy already has a fighting chance of defeating the most powerful out of the four Yonko. What Kaido fears about Luffy in this scene is that the rubber boy could potentially surpass all five of the other men who have stood on equal footing with him.
Seeing such a powerful antagonist shudder for the first time in front of Luffy is breathtaking, and learning more about Kaido and his shared past with Whitebeard, Roger and Rocks will no doubt be an important moment in One Piece's unfolding timeline.