When I have brought them to the land flowing with milk and honey, of which I swore to their fathers, and they have eaten and filled themselves and grown fat, then they will turn to other gods and serve them; and they will provoke Me and break My covenant. (Deuteronomy 31: 20)
In an ironic way, it is God who prophesies to Moses. He forewarns Moses that after the Israelites have been delivered and enter the Promised Land, they will eat themselves to the full and become fat. In their fatness, they will turn to other gods, serve them and provoke Him.
Getting fat means more than gaining weight. It means being full, but not fulfilled. It means ingesting, but not digesting. It means being saturated, but not satisfied. It means getting bigger, but not getting better. It means having more exposure, but not necessarily learning from experience.
As it relates to our spiritual maturity, let us not substitute Christian activity for Christian maturity. Let us not mistake swelling for growth. Let us not equate knowledge with understanding. Let us not become so satisfied with where we have come from that we no longer pursue what we can yet become. Let us never get so fat that we forget and forsake the God who brought us and delivered us.