Paul warned against idleness in 2 Thessalonians 3:6–12 (RSV), where he emphasized the importance of hard work.

• He reminded the Thessalonians of his own example, stating that when he was with them, he worked day and night so as not to be a burden to anyone (verse 8).
• He also said, "If anyone will not work, let him not eat" (verse 10), reinforcing the need for diligence.
• Paul did not just teach about hard work; he demonstrated it through his personal experience as a tentmaker (Acts 18:3).

Since Paul used his personal experience to teach against idleness, the correct answer is D. his personal experience.