Against preparing sermons, some preachers used to quote the verse, “Open your mouth wide, and I will fill it” (Ps 81:10). Or perhaps I should say more precisely, they used to quote a single line of the verse. Their resistance to preparation apparently extended to examining the context of the lines they quoted.
In the verse itself, God reminds them that he redeemed his people from slavery in Egypt and spoke to his people in the wilderness (81:8-10). His people resisted obeying him, following other gods, so God punished them (81:11-12). But if they would obey him, he would bless them, as he wanted to do (81:13-16).
What would he fill their mouths with? In this case, not what should come out of the mouth (words) but what should go in (food). As God provided manna for his people when they were in the wilderness, so he longed for their obedience so he could bless them with food:
“But you would be fed with the finest of wheat;with honey from the rock I would satisfy you” (Ps 81:16, NIV)
The verse is about provision, but not about providing sermon material without study, when we have access to be able to read the Bible. It’s about God supplying the needs of his people when they follow his ways.