Greatest, not only.
Well He also did say that ALL the law and the prophets hang from those two greatest commandments.
Let's put it this way. There's a line to Saint Peter's desk. One by one each soul reaches him, and he barcodes their thumbprint on his iPad and calls up their file, and reads it. "Murderer. Nope" he says, and taps the iPad, and a door opens up under their feet and they drop away to hell. "Rapist. Nope." Tap, they're gone. "Thief, Nope." Tap. "Politician. Nope." Tap. "Advertising Executive. Nope." Tap. "Telemarketer. Nope" Tap. "Child Molester. Nope." Tap.
Then I get up there. "Hmmm, lemme see. You loved God with all your heart and all your mind and all your soul? Check. And you loved your neighbor as yourself? Check." Saint Peter the looks at me and squints. "Anything else? You noticed the Bible is more than one page long, right? Well?" He waits. "Nope" he says, and reaches to tap his finger.
"PETER!" A voice booms from the sky. "KNOCK IT OFF!"
Sheepishly, Peter pushes a button on his desk, and the Pearly Gates open wide.