After delivering the Israelites from slavery in Egypt, God had to teach them how to live and journey with Him. Centuries of bondage had taken a toll on His chosen people, and He worked patiently to turn their hearts to Him - despite their chronic rebellion and incessant grumbling.
So how could the Israelites possibly question whether God was among them? They knew without a shadow of doubt - they had seen His presence, heard His voice, and eaten the food He provided. Yet they were still inclined to act as if they had never gotten the memo!