Family, study, work, and friendship all involve reliance on what others say. Deliberately lying when someone trusts you does double harm: it conceals truth and damages that trust.
Example 1: Someone obtains a loan by deliberately misstating their plan or ability to repay.
Example 2: A teacher confidently asserts a matter without knowledge merely to avoid admitting uncertainty. Responsible teaching requires recognizing one's limits.
مُصَدِّقٌ (“one who believes”) and كَاذِبٌ (“one who lies”) are active participles. Their contrast brings the moral problem into focus.