Observe that when Jehoshaphat sought God and “walked in His commands,” God “established the kingdom in his hand.” Seeking and walking in God’s commands resulted in his being established. Do you want to be established, to last? The example of Jehoshaphat’s life teaches that you should seek God and conduct yourself according to His commands and not according to the culture surrounding you.
Contrast that with Ahab, who did more evil than all who were before him. That’s how sin works: it doesn’t get better or maintain a status quo, but it grows worse and worse. Decay is a facet of corruption. You aren’t allowing corruption to be part of your life, are you? Because the reality of knowing Jesus as Savior gives you a choice in that matter, since He frees us from sin.
Jehoshaphat didn’t just remove the high places associated with idol worship; he attempted to re-direct the hearts of the people by sending officials, priests and Levites to teach the people the Law of God. Removing sin doesn’t just mean getting rid of something; the resulting void needs to be filled with something else. Recall Jereboam’s efforts to direct his people’s hearts away from Jerusalem: he presented a new religion complete with new observances and holidays. He certainly tried harder to lead his people away from God than Asa tried to lead them back. Our enemy is passionate about luring souls away from God; his passion must be matched and exceeded by a desire to win souls back to God. God has demonstrated that passion by coming to earth, suffering and dying for lost souls. Do we share His passion? That is our calling.