Original: Ahab allowed the worship of a foreign god within the palace, building a temple for Baal and allowing Jezebel to bring a large entourage of priests and prophets of Baal and Asherah into the country. It is in this context that Elijah is introduced in as Elijah "The Tishbite".
Simplified: They argued, and Elijah told Ahab to "summon the people from all over Israel" to meet him on the Mount Carmel, and to "bring the four hundred and fifty prophets of Baal and the four hundred prophets of Asherah, who eat at Jezebel's table".