Why are good, religious people the hardest for God to save?
Good, religious people are the hardest for God to save because they are so good and so religious that they see no need for any salvation. Yet they still need it. Their own goodness and religious devotion convince them they lack nothing, so they do not seek the salvation God provides.
Les explains that the hardest person for God to bring to salvation is this kind of person: someone who is so good and so religious that they see no need for any salvation. But they need it, and so did Cornelius. Cornelius prayed and gave alms, and his prayers and alms came up for a memorial before God, yet he could not call God Lord by virtue of a personal experience or relationship with Him. It was just a term. He had to send for Peter, who would tell him what he ought to do. So religious goodness and devotion do not remove the need for salvation; the very fact that such a person senses no need makes them hard to reach.