Percy Jackson and the Olympians season 1 finale, "The Prophecy Comes True," let us and Percy in on who really stole the Master Bolt. He'd been working on the conclusion that it'd been Ares and Clarisse but that wasn't the case. He found this out when he was lured into the woods by Luke who'd said they were going to be talking to Chiron about what they knew. That, however, wasn't why they were there.
Luke wanted to show him Backbiter, his sword which can make a tear in their plane of existence to create a doorway. The older demigod had been working with Kronos the entire time. Tired of the tragedies and heartbreak caused by the Olympian gods who've been terrible parents in the grand scheme of things, he wanted to bring forth the "Golden Age" as that's what Kronos' rule had been called.
Percy was gobsmacked. He couldn't believe that Luke had fallen for Kronos' logic. He tried to get his friend to listen to him but as soon as he mentioned Hermes, Luke attacked. They fought, swords clashing, but Luke overpowered Percy after the younger demigod landed a slash against his chest and apologized right after. Using his distraction against him, Luke slashed at his hand which caused Percy to fall.
If it weren't for Annabeth, Percy might have met the end of Luke's sword. She threw her dagger at her brother's weapon. He was thrown off by her involvement and her statement that she'd heard everything he'd said. Instead of talking things out or arguing his point even more Luke leaped through the hole he'd created with his sword and left camp like he'd tried to get Percy to do with him.
Obviously, this isn't over. Luke told Percy that he considers him a friend and planting the bolt on him wasn't meant to be a betrayal. It was simply a part of the plan. He actually wants to recruit Percy to his cause which is why Chiron cautioned Percy as he's now become a leader of his fellow demigods. If Percy were to cross to Team Kronos' side, he wouldn't be alone. Some demigods would likely follow him which wouldn't be good for the Pantheon.
But for now, Luke is out there possibly recruiting disillusioned demigods like himself into a fight that can only end badly with Kronos involved.