The first commandment with promise (entolē prōtē en epaggeliāi). En here means “accompanied by” (Alford). But why “with a promise”? The second has a general promise, but the fifth alone (Exo 20:12) has a specific promise. Perhaps that is the idea. Some take it to be first because in the order of time it was taught first to children, but the addition of en epaggeliāi here to prōtē points to the other view.