Alma sees “all things” as testifying of Yahweh’s existence (v. 41), while Korihor, with access to the same evidence, continues to deny Yahweh. In verse 42 Alma levels a terrible accusation: Korihor is denying Yahweh even though he really does believe. Alma must have had confirmation from the Spirit that Korihor was in such a state, although he may have deduced it if Korihor were an apostate Nephite. Korihor has “put off the Spirit of God that it may have no place.” Alma depicts Korihor as someone who began with faith, most likely relying on his parents’ faith and the faith of his youth. He has actively repudiated that youthful understanding, converting to the more secular Lamanite philosophy. He has been able to do this because he changed his definition of what constitutes evidence, or proof, of God.
Alma’s strong evidence for understanding Yahweh and the Messiah are all based on the Spirit, the only true source of such knowledge. To deny Yahweh, Korihor has denied the Spirit and thus denied his only possible source of that knowledge. Alma is suggesting that Yahweh cannot be reached through a scientific or logical approach but only through the Spirit. Korihor’s denial of the Spirit has changed his definition of what constitutes acceptable evidence, thus leading Korihor away from the very things that would have given him knowledge of Yahweh.
Worse, Korihor’s denial of the Spirit in his own life has led him to persuade others to also deny it. Such behavior is frequent among apostates. As Joseph Smith found out early, some of the most difficult pressure against the Church came from those who had once embraced it, then left. Once again, Korihor’s strong motivation to lead others down his path suggests that he had earlier been a Nephite believer.